There are four Classes in Elder Scrolls Online, each providing 3 Class Skill Lines and a total of 18 Active Abilities and 12 Passive Skills.
Your Class, along with your Race and Alliance, is a key trait of any character. Any Class and Race combination can play any role (damage, healer or tank), but some Classes and Races are suited to certain roles and will finder it easier to perform in those areas.
Your choice of Class will only determine your Class Skills, you can use any weapon, any armor type, and many other Skills regardless of your Class. At this time it’s not possible to change your Class, but you can always make new characters to experience all of the choices.
Dragonknights are a good all-round Class, with skills for dealing damage (especially Flame Damage) and tanking. The three Skill Lines for a Dragonknight are Ardent Flame, Draconic Power and Earthen Heart.
Nightblades are great rouge-like characters with the only invisibility spell in the game. They also make good damage dealers (especially melee range) and have a number of Stamina morphs. The three Skill Lines for Nightblades are Assassination, Shadow and Siphoning.
Sorcerers are great at ranged damage and mobility. They boast a teleport skill, a ranged execute and many lightning spells. The three Skill Lines for Sorcerers are Daedric Summoning, Dark Magic and Storm Calling.
Templars make great healers – that is probably their defining attribute with an entire Skill Line for healing and support. They also have a number of tanking skills and a channelled ranged execute. The three Skill Lines for Templars are Aedric Spear, Dawn’s Wrath and Restoring Light.