From Destinypedia, the Destiny wiki
Subclasses (formerly called focuses) are the primary way Guardians improve and level up. Each subclass advances independently from level, although until level 15+ these upgrades synchronize with level. New abilities, passive bonuses, grenade types, and more unlock as the subclass gains experience, with each subclass containing twenty-eight upgrades. Subclass abilities are shaped as the player levels up, and each upgrade tree is distinct. In any given subclass, the player is offered a number of tiers that give the player a choice of attributes such as:
With the exception of the three "base" abilities (jump, Super, and melee), each column of three upgrades is mutually exclusive; only one can be active at a time.
Every class has two subclasses to acquire and develop, each with its own specialties. Subclass is effectively a piece of equipment that can be changed at will, much like selecting a new weapon. One subclass can be used as a competitive multiplayer loadout, while a second healing-oriented focus can be used for cooperative play. Subclasses are also meant to be synergistic with the gear the Guardian is wearing.
Upon mastering a subclass, the player is awarded an Exotic Weapon Bounty from his or her Vanguard, as well as an achievement/trophy.
In Destiny 2, subclasses were overhauled, with the implementation of level trees taking the former system's place. Guardians have the choice of selecting one of two different ability focuses for each subclass, being locked into that particular path.
List of Subclasses
List of Appearances