Stop motion animation or model animation is another type of classic animation. The objects you are animating are real, three dimensional figures or objects. You position them, shoot a frame of whatever animation medium you are working in, make a change in the objects, and shoot another frame. Over and over. Your character sculptures are most often smaller than life size, and may be built on armatures that give them a definite "skeleton". The armature helps you create realistic (or at least believable) motion, and provides support for the body and accessories of the figure.
Stop motion animation has mainly been used to create character animation but has been used for abstract animation as well.The original King Kong (1933) and all of Ray Harryhausen's movies featured stop motion animation. Tim Burton's "Corpse Bride" is a spectacular modern example. Here's an older movie, "The Milky Way", that I came across in some old 16mm films: it's one of a series of movies from Kinex Studios, made in the 1920s.
The insect models in this movie are larger than life size! The rules are, there are no rules.
Modeling clay is a favorite medium for stop motion animation. Jimmy Picker won an Academy Award in 1985 for "A Sundae in New York". Here's another of his movies, "Breakdance".
Stop motion animation is the second most common kind of classic animation. It puts fabulous creatures into live action movies (King Kong, Harryhausen's movies) or creates memorable characters and stories in worlds of their own (Corpse Bride, Chicken Run). It's a lot of work, creating a whole world, all the characters in it, and then acting out the movie one frame at a time. Some types of computer generated (CG) animation look a lot like model animation. And of course people are combining the forms as their artistic whims dictate!