An eldritch blast is a warlock's most useful natural ability. Eldritch blast is treated as a ranged touch attack (and as such, your Dexterity factors into the AB).
By default, eldritch blast has no specific type and is limited to a range of 30', with no saving throw (but spell resistance applies), and has an effective caster level (ECL) of 1. Eldritch essence & blast shape invocations will modify the effects of the blast. This usually increases the ECL. If it has an additional effect that applies a save, your Charisma modifier will be applied to the DC.
Although an eldritch blast of 10d6 or greater will bypass Spell Resistance checks and spell mantles, only base/unmodified d6 blast levels qualify towards this end; Hellfire Warlock levels don't count. The ability to bypass SR and spell mantles appears to be because the game recognizes the spell level of the Eldritch Blast by its base/unmodified damage die; once this exceeds 9 the spell resistance code breaks, as it wasn't designed with spells higher than level 9 in mind.