A computer is...think of a floo connection, but in a very small box and it can connect to every other box in the world like it. The box shows you pictures, lets you send messages, let's you talk to people, see them.
I'm not really good at them, but I've seen them at Rose's and they are awesome. You know, I read that Cho lived in the Muggle world. Maybe you should get her to teach her since she'd have to use them daily, while my knowledge is more watching others.