Page 101 of the PHB defines cantrips as:
A spell that can be cast at will, without using a spell slot and without being prepared in advance. Repeated practice has fixed the spell in the caster's mind and infused the caster with the magic needed to produce the effect over and over. A cantrip's spell level is zero.
Although a cantrip can be cast over and over, they do have limits. Repetitious use of any cantrip can lead to exhaustion. Every 10 rounds (1 minute), the caster must make a DC-12 CON roll. On failure, one exhaustion point is earned. A long rest (plus food and drink) is needed to remove one level of exhaustion. In non-combat situations, a player may cast 1 cantrip per ten minutes without needing to rest. This may vary a bit for some cantrips, such as Mold Earth.
In the Nolaria Campaign, cantrips fall into two categories: strong and weak. Strong cantrips are those defined in the 5e rules. Weak cantrips are unique to Nolaria and are generally far less powerful that strong cantrips. Examples include:
Trigger - used to trigger previously prepared spells, usually as runes.
Repel Small Insects - keeps minor insects, bees, etc. at bay.
Turn Page - turn a single page in a book forward or backwards.
Clean Clothes - instantly causes all dirt, grim, stains to leave clothing.
(there are many others)
In certain casts (criteria is TBD) non-spell caster NPCs can learned to cast lesser cantrips. In some parts of Nolaria, for example, there is something called the Talented Housekeeper who has the talent for casting housekeeping cantrips, such as Clean Clothes (above). Certain weak cantrips are also used by Cabbalists and rune lore, such as the Trigger Cantrip.
The following character class can use cantrips:
Bard
Cleric
Druid
Figher - Eldritch Knight
Rogues - Arcane Trickster
Sorcerer
Warlock
Wizard
Weak cantrips are not counted against the number of cantrips known by the classes above.