Stop-motion animation is an incredible thing. When you first think of what it entails, the effort involved seems almost super-human. Let me explain.
How many frames are there in an average film? Probably about twenty-four a second, for an hour and a half (average film length). That's a hefty 129600 frames. Now in stop-frame animation, every single one of those frames is an individual photograph, taken by an animator. Each model, usually fashioned from clay, is repositioned just the tiniest of amounts between each shot. When these photos are run together as a film, persistence of vision makes them appear to be a smooth animation, just like a cartoon or a normal film.
Notice I said "just like a cartoon". There are a lot of similarities, indeed: from the individually-created cells forming a film to the arduous repetetive tasks, there is a clear parellel between the two. However, there are also differences. Disney animators have at their disposal all manner of computers and such like to take some of the "leg work" (hand work?) out of animation - and also create some stunning new effects. For example, the stampede in The Lion King and Tarzan's endless forests. On the other hand, it's hard to argue that the actual animation process of stop-frame has evolved a great deal since Ray Harryhausen. The quality has gone up, because budgets have, and of course so has individual skill of animators given more time to practice their art. Nevertheless, there's little qualititative improvement to speak of.
A lot of people ask how stop-frame can still have a place in our age of computers. You could equally argue against the role of cartoon animation, of course, but Nick Park of Aardman Animations, creators of Wallace & Gromit articulates it best when he says, "Stop-motion is an outdated media but that's the beauty of it. We have cameras but we don't stop painting portraits." With the pressure to push technical boundaries removed, the artistic side of films is able to flourish - Tim Burton created something really incredible in The Nightmare Before Christmas for just this reason.
Stop-frame animators have another disadvantage compared to their cartoon-based counterparts. A cartoonist can see all the frames of his work as he goes. He can flip backwards and forwards to compare the evolution of frames at will. In stop-frame however, the only available shot is the last one (the so-called "reference shot") and the model in front of him. Therefore, it requires a great deal of patience and skill to maintain the gradual progression of motion across shots that the final film requires.
The great advocate of stop-motion was Ray Harryhausen, who created almost all of the great dinosaur-type movies of his era, not to mention the special effects for such classics as "Clash of the Titans". Held up against more recent productions, his animation seems very jerky and the models themselves are not up to modern standards, but he was really a ground-breaker and left the refining of the art to others. This challenge was taken up, for one, by Nick Park; the creator of the famous electricity adverts that everyone thinks are for British Gas with the talking tortoise and others. He then created the Wallace & Gromit trilogy, and finally Chicken Run. That's pretty much the epitome of full-length stop-motion films currently, although a Wallace & Gromit movie is rumoured to be in production.
That was a brief history, explanation and indeed something of a validation of stop-motion animation. I feel much better for writing it too.