The first keyframe animation has turned 100 today. It's Gertie the Dinosaur, created by Little Nemo in Slumberland's Winsor McCay. You can see the whole thing below, including the live action meta-narrative that surrounds the animated portion.
Beautiful stuff. 100 years old and as entertaining as ever.
And so we were introduced to creating animation by drawing individual images in sequence, working between "key frames" that hold the key poses and delineate the progress of motion. I don't think we could easily overestimate the impact of this technique on cinema.