First of all, it's almost impossible to answer that question neatly. As the cinemaniacs at Not Coming to a Theater Near You say right off, there is an "unimaginably large spectrum of films" that fall within the "animation" definition. So rather than deliver a long-winded response, these movie buffs have pulled together a series of articles on "individual cases" they see as significant. Lively essays on such commercial whizzes as Walt Disney, Hayao Miyazaki, and Pixar brush up against paeans to odd and forgotten types like Czech stop-motion master Jiri Trnka and the masterminds of live-action puppet films. Individual examinations of "The Triplets of Belleville" and "Fantastic Planet" waltz through. And, because it wouldn't be a proper party without them, the wild boys of "South Park" get their honorary nod. For 10 years, they've violated "all boundaries of accepted decency on televison" and still managed to be one of the funniest things out there. Perhaps more important than "what is animation?" is "where would we be without it?"
