WoTC vs. Chaosium:
Call of Cthulhu is a game originally (and still) published by Chaosium, using their Basic Role Playing system (still available from Chaosium for about $6.00). Some modifications have been made over the past 20 years, but most of those were minor fixes or clarifications. Currently in it's 5th edition, the CoC rules are complete in one book.
There are no classes, but rather occupations that have certain grouped skills in them. Occupation also determines your income. Your skill points are determined by your stats. All skills are percentage based; you might have Shotgun at 50%, Speak Latin at 20% and Sneak at 15%.
WoTC did the d20 version, and a nice conversion it is, too.
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Call of Cthulhu deals with investigators - mostly normal people from everyday walks of life - that become emmeshed in something terribly and horribly alien. They may seek out these mysteries for many reasons, but usually once 'exposed' to the Mythos, one rarely has any sense of peace of mind, ever again.
Draws from the themes of Lovecraft's fiction: alieness, the fear of the outsider, the helplessness of Man vs. a cruel and uncaring Universe.
Mostly, investigators encounter occult mysteries based around one or more cults, monsters or - rarely - the great powers themselves: the 'Gods' of the Mythos. The Gods are really alien beings so vastly intelligent and powerful that we are as nothing to them.