Did you ever have the deck of cards which when flipped showed an object
moving?
Like a movie, animation is just a series of still images played in sequence.
While Disney Studio used to draw such pictures by hand, now a computer process called "in-betweening" is used.
Macromedia Director is a multimedia presentation package which performs in-betweening to produce animation.
Animation can be:
Server Push/Client Pull Animation for the Web