Illusion covers the range of abilities which allow the caster to conceal themselves, and illuminate dark areas. Illusion is also used on targets to charm, silence, paralyze, frenzy, or demoralize; on the other hand, it can be used to rally targets as well.
The sheer number of effects through which the caster can affect targets through Illusion alone makes it a versatile skill which can serve the needs of mages and fighters alike. In The Elder Scrolls V: Skyrim, Illusion spells will remain effective on higher level targets including the Undead, Daedra and otherwise "mindless" beings like automatons, but only if you select the right perks.