Here's what I understand to be correct:
A forkball, thrown correctly, should have forward spin, and the harder its thrown the better. The ball will begin to spin on the middle knuckles of the hand before it is actually released! This is a true breaking ball, and normally is thrown with a bit higher trajectory than a change or fastball to accomodate the drop. The arm motion is very fastball-like, which helps disguise the fact that its a breaking ball. I've thrown a forkball and I can verify that it behaves this way when thrown correctly.
A splitter is thrown like a fastball, too, but the good splitter has very little spin. Its travels faster than a changeup because its not choked into the hand, but with little or no spin it will die at the plate. Like Roger says, its considered a changeup variant by most folks I know.