There are two main types of drawing tablet: display tablets and graphics tablets. Display tablets have a built-in screen, while graphics tablets are drawing surfaces that require an external display such as your computer's monitor.
Display tablets are the better choice for animation where possible, as having your drawing surface and your display in the same place makes for a smoother experience, and makes it easier to take advantage of the various animation software programs.
However, they also tend to be more expensive than graphics tablets – and if you already have invested in a high-quality monitor, a graphics tablet may be the more cost-effective choice.