Typically in animation we see credits for key frame animation and inbetween animation.
A key animator is the artist responsible for producing the principal key frames of an animation. Basically key animators draw the essential frames that mark a distinct position or expression of a character in a particular scene. In other words, they draw the structure of a animated scene.
In order to complete the animation and make sure it moves fluidly, additional frames are required to go between key frames, this is called in-between animation.
In the credits of a lot of anime nowadays you see something called "second key" animation. What purpose do they serve in an anime production?