There are three kinds of tablet: graphics tablets (connect to a PC or Mac), pen displays (drawing onto a screen) and computer displays (as the name says, a computer and display combined). All are good for animation, but go up in price the more tech is on offer.
If you can afford it, a computer tablet is excellent – this can be an iPad Pro, Wacom Cintiq or Huion Kamvas Pro. The good medium ground is a pen display that uses a computer to power the software but enables you to draw onto a Full HD screen.