Templars are typically healers, due to the multitude of buffs and supporting effects they can bestow upon the party, but can also serve as very capable damage dealers of both kind and decent tanks.
The game makes a point that all classes can be utilized for every role, though some tend to perform better than others in certain aspects. The three core roles are tanking (taking damage so others do not have to), healing (patching others up), and damage dealing, which itself is composed of two subcategories: stamina and magicka. The Elder Scrolls Online's classes follow the standard role-playing archetypes.