I learned it this way:
Warlock; A spell-caster that uses magick to aid himself or others during some form of conflict, usually a physical fight of some kind.
Sorcerer; A spell-caster that was born with a strong talent for spell-casting, usually surpassing others who study the same fields of magick.
Wizard; A spell-caster that was not born with a talent for spell-casting, but has learned to cast spells through allot of research, study and practice.
But these may not be correct. I don't hold any of the titles because I do not cast spells. :) I do perform rituals on occasion, talk to Spirits, and manipulate energy, but I wouldn't call any of that spell-casting.