The following comes from the the Heralds of Valdemar Wikia.
Heralds are exclusive to Valdemar, possess the highest quality of moral fiber and exist only by virtue of being Chosen by a Companion. Each Companion is destined to Choose someone, and most do so well in advance of reaching ten years age, though occasionally the wait can be quite long. (Yfandes was ten years of age when she Chose Vanyel Ashkevron, shortly after the death of his lifebonded, Tylendel.)
When a "youngling" or young person is chosen, the Crown may, if its family is poor, provide a stipend to cover the labor that person might have provided for the family. It is perceived as an honor to have a family member "Chosen".
Heralds are Chosen because they have the ability to contribute to Valdemar, or on occasion, because they have exact skills which are desperately needed by the kingdom at the time. There is often a certain wariness that is brought on when a large number of new Heralds are Chosen, as this means one of two things: that Valdemar will be expanding in coming years, or that there will be a significant conflict or trouble in the coming years. Both of which would require more Heralds. Historically, the latter situation is more common, especially when the newly Chosen possess certain Gifts that are more suited to battle rather than diplomacy.
Heralds are often chosen when their special Gift emerges; although usually chosen as young adults, a person of any age might be chosen. It is a violation of Valdemar law to prevent a Companion from accessing its Chosen.
Those who are chosen have at least one Gift which is psychic in nature (the Mage-Gift is special, and restricted in the stories to the Ashkevron line (including the royal family) after the 500 year loss of magic following the death of Vanyel Ashkevron); training completed at the Colleqium and through mentorship by fully trained Heralds, as well as provided by their Companion, enhance and develop the Gifts to their maximum which varies from Herald to Herald. Additional gifts may arise during or after the development of the first Gift.
All Heralds start as Trainees, and learn law, languages, weaponry, strategy, geography, science, survival skills, orienteering, horsemanship, surveying, maths, literature and sometimes specialist topics such as disguise, spycraft, or how to use their gifts in a specific manner. Trainee Heralds wear gray; fully trained Heralds wear a white uniform sometimes referred to as "Oh Shoot Me Now" whites due to their conspicuous and visible nature.
Past Herald-Trainees are noted as including assassins, spies, foreign agents or military personnel including officers, and a host of other unexpected things, and in Take a Thief, Skif becomes the first Herald-Trainee to have been an actual thief, though Donni was an apprentice thief.
The average age of a Herald-Trainee is usually twelve or thirteen, though Trainees have been known to be as varied in ages as in background. After anywhere from three to five years worth of education, the Trainee is then granted Whites and sent out on an internship of rounds with a more experienced Herald which lasts from a year to two years. Exceptions are usually made for some Trainees, including the Heir-Presumptive, who usually undergoes a long term internship serving in Haven's Courts and other duties within Haven, and those who have certain special abilities which cannot be risked, including Herald-Mages.
Certain roles within Valdemar, such as being Monarch or the Monarch's special advisor ("The Monarch's Own") require that the holder be a Herald, thus a presumed heir to the throne must be "Chosen" to be an heir.
Heralds all form strong mind-to-mind bonds (called "Mindspeech") with their Companions, even those who are not able to speak mind to mind with anyone else. The Companions use this to maintain a web of communications keeping all Heralds and companions linked.
Heralds are called "The Arrows of the Queen," and serve the Kingdom of Valdemar and its Monarch as messengers, spies, information-gatherers, peace-keepers and civil marshals. They also serve as leaders in times of crisis. They serve on "circuit," where they cover a section of Valdemar to resolve civil disputes, support the Guards in maintaining order, uphold Valdemaran law, and act as the representatives of the Crown.
Fair, incorruptible, self-sacrificing, and above question due to the mystical nature of their Companions, the Heralds are respected throughout Valdemar and feared by her enemies.
Heralds are held to an extraordinarily high moral standard. If a Herald or Herald-Trainee does not abide by this moral standard, they will be hounded by their Companion until they fall into line, and one who does unforgivable acts will generally be repudiated by their Companion. This will usually be followed up by the Companion's suicide in some manner or another. The result for the repudiated Herald or Herald-Trainee is generally a wound so deep that it may mar the soul, though that is not assured, but it will also quite commonly leave the victim catatonic as a result, blankly nonresponsive. Those who are responsive, are typically insane to the point that they are unable to use any Gifts that they might possess. However, repudiation does not necessarily mean that one is relieved of their Herald status, as when Tylendel was repudiated, the Death Bell tolled for him, indicating that he was accepted by the Companions as a full Herald and should be buried with Herald's honors. One can assume that Tylendel was given this distinction because his repudiation and the terrible acts that caused it were in turn caused by severe psychological trauma.
Companions
Companions are highly intelligent magical creatures who look like (but are not) horses. They are easily recognized by their pure white coats, bright blue eyes, and the sound of their silver hooves, most commonly referred to as the sound of bells. Each Herald has such a Companion. Each Companion is white because of their connection to Node magic. They draw upon Nodes to enhance their abilities, giving them extended endurance, greater speed and the ability to Heal themselves quickly. For example, in cases of extreme urgency, Companions use the nodes to make short "jumps" in space --- an ability Yfandes describes as a "sort of Fetching" except with themselves and their rider--- which when coupled with their speed allows them to cover great distances in a small fraction of time. Some Companions can also enhance the abilities of their Chosen, feeding them energy from nodes and ley-lines. Many Companions are reincarnated Heralds.Companions also seem to have a deep connection to the Star-Eyed Goddess, much like the Firecats have a connection to Vkandis.
Companion's Choice
Companions "Choose" Heralds in the trainee or apprentice stage; the exact nature of the "Companion's Choice" is not stated explicitly in the novels. Heralds have described the moment in various ways: "being weighed and measured," "falling into [the Companion's] blue eyes," and "finding something I didn't know I was missing." The defining factor of one's Choosing seems to rely on the individual's moral fiber and where applicable, their ability to provide something that is needed by Valdemar, at the time. The Choice bonds Companions to their Heralds into a mutual relationship, and the bond can affect both parties' spiritual and physical well-being. The Companion becomes a loving friend who guides, supports, and sometimes augments a Herald's abilities. The joining is known to strengthen the Herald's Gifts, especially Mindspeech.The Choice is, for the most part, permanent; it can be broken while in its early stages, although doing so will physically and emotionally scar both parties. After the initial phase, however, the bond becomes so important that the loss of either Herald or Companion is followed by depression and almost always death. Despite this a Companion can break the bond with their Chosen if at any time, they believe the Herald or Herald Trainee to have become unworthy. This is called repudiation. The most famous case of this was Tylendel, a Herald-Mage trainee and Herald-Mage Vanyel's first lifebonded. (Magic's Pawn)
Many Heralds can mindspeak with their Companions, but not all. The only time that a Herald might hear their Companion is the moment of choosing. Heralds Sherrill and Talia only heard their Companion's voice when they were Chosen. Herald Sherrill couldn't communicate with her Companion at all after that.
Grove-born Companions
The Companions first appeared from a grove near the palace in Haven, seemingly in response to King Valdemar's prayer, though there are indications that the king, also a mage, may have cast one or more spells as well. Though the first Companions were all Grove-Born, later generations were born in the manner of normal horses. In "present-day" Valdemar (Reign of Queen Selenay), only three Companions are revealed to be Grove-Born: Gwena (Companion of Herald Elspeth), Rolan (Companion of Talamir and later Talia), and Rolan's predecessor Taver (Companion of Talamir).Though the true nature of the Companions has not been fully revealed, it is inferred they are both the returned spirits of departed Heralds and Companions, as well as divine agents or avatars of the gods. In the novel Oathbreakers, the Shin'a'in goddess specifically refers to a Companion as the "child of my other Selves". That being said, Companions take great pains to reveal that they are both fallible and mortal. When a Companion or Herald dies, he or she is sent to a "resting place" where they can recover from their recent life. After a time they are given the choice of returning as a Companion, with all memory of their past life intact, or to lose all of their old memories and become a human baby destined to become a Herald. Not all Heralds are returned from previous lifetimes, many are "new souls" who are then Chosen by the Companions to join the ranks of the Heralds.
These spirits are not allowed to return as Companions within the lifetime of the people they knew to avoid emotional entanglements such as Choosing their own child, or revealing themselves to their former lover. When they return as Companions they often use some variation of their former human name (for instance, a Herald-Mage called Savil might use the name Sayvil as a Companion).
The Companion to the Monarch's Own Herald is always a Grove-Born Stallion, and is considered the leader of the Companions. This Companion, currently Rolan, has the ability to bond with a second Herald if the Monarch's Own dies; as Rolan has Chosen both Talamir and Talia (Arrows of the Queen). The Monarch's Own is also subject to this; if his or her Companion dies, it is possible for the Herald to bond to a newly-arrived Grove-Born Companion (Exile's Honor). Also, presumptive Monarch's Own who are also Heralds are only temporarily bonded to their first Companion; though the exact nature of this bonding is not explored (Magic's Promise). The Monarch's Own Companion is not a reborn Herald, They may be a true Avatar. There seem to be several who take turns. Rolan was Monarch's Own Companion during the Collegium Chronicles and Brightly Burning, two hundred years later Taver has replaced him, and Rolan returns when Taver is killed in battle.
The only Grove-Born mare since the time of King Valdemar is Gwena, who seems to have been "newly created" for the purpose of being the Companion to the first Herald-Mage in four hundred years. Being Grove-Born means she does not suffer some of the same limitations as other Companions. For example, not only can she feed Elspeth mage-energy, she can do so over any distance. There is no such thing as too far away from her Chosen.