Stop-motion animation (or stop-action) is the painstaking process of photographing a model, moving it a miniscule amount, then photographing it again. Finally, you string the photographs together and the tiny movements appear to be action. This form of animation is the simplest to use and is great for beginners.
For instance, Seth Green, an actor who has a love of action figures but no prior animation experience, co-created Robot Chicken with Matthew Senreich. They employ toys, sets that are more like dioramas, dollhouse props and clay (for facial expressions) in their stop-motion videos to create some pretty hysterical skits.
Though I say this technique simple, because the concept is easy to understand and execute, that does not mean stop-motion is not time-consuming or cannot be sophisticated.
In the hands of an artist, stop-motion animation can be very realistic, stylistic and moving. Films like Corpse Bride by Tim Burton show that stop-motion isn't a genre, but a medium that allows artists to create whatever they imagine. Each character in this film has several versions of bodies and heads in order to capture the most human movements and expressions. The sets are also created with the same attention to detail, creating a dark, beautiful world.