Collecting animation art isn't the most normal of hobbies. I suspect that people are led to doing it by a combination of three things happening at once.
It is all to easy to spend a fortune on the stuff which is a shame really because it's all very pretty.
People usually first see animation art in Disney or Warner Brother's shops in the little quiet gallery bit at the back. Most of these are not actually original art at all but are copies of famous scenes known as Sericels or sometimes even scenes specially created for collectors. Very few serious animation art collectors will have more than a few of these in a collection and I think I have about three. It really boils down to being a personal thing; I don't really see the point of collecting something that other people have and Sericel collectors don't see the point in collecting used and often quite low quality animation cels because they will rarely get the really good scenes they want and usually have to pay a lot more.
I tend to collect original cels, which are the actual "moving bits" of a cartoon; I try to collect original backgrounds, which are the hand painted sets that the characters interact on; I have quite a few sketches, which are the drawings the cels are painted from and I have a couple of storyboards which basically are pictorial scripts for a cartoon.
I am a sucker for Hanna Barbera art, there seem to be loads of signed peices out there which makes me think that the two of them must have spent years just signing things. The Top Cat setup at the top of this page is a very nice signed one and good rugrats setups (cels with backgrounds, though the background is usually a copy of the original) are usually signed as well. Signing in this field is often a bit weird, I have a few signed Matt Groening peices (he created The Simpsons) but it is unlikely that he actually drew most things he has signed and he admits that certainly in the case of The Simpsons, he wouldn't get a job as a Simpsons animator any more because he draws them so differently than their modern looks. Still, the signatures are sweet, and they usually show that the peice is considered to be a good enough peice of art to actually bother signing. I do collect Disney art but the stuff I like, I can't afford.
As more and more cartoons are becoming computer animated there is less and less colour artwork available, it's still possible to get some sketches but even these are getting less and less. Luckily, most modern cartoons are crap anyway so this isn't a problem and there is still tonnes of good old stuff out there.
Until I format this better, I will just use Webmagick to show some examples on here, there's more than one page so use the little arrows at the top as well.
Should you want a much more detailed description of the types of animation art, take a visit to Debbie Weiss' pages at http://www.animationartcollecting.com/ - She runs The Wonderful World of Animation which is a very good place to start collecting, unlike some it's not hugely overpriced and the staff are great.