1) An Avatar is a computer generated image that represents a person either being you or just lokking like anything you want.
2) They might use it in personal life to just have fun with it or to puit on a blog. As a teacher you could use an avatar to teach students in a fun new way. You could even use it in your business to put on a website and tell people what your business is about.
3)Yes for some games I have made avatars that you use to play with.
4) I liked clay yourself the best it seemed easier to use and there was a gallery of everyones avatar. But in GoAnimate you had to pay to use a lot of things. And the Voki just didn't have as many things that you could do to it like add a hat or special things like head phones.