An ANSI-bomb changes the keys of your keyboard. For example, when you press return it will ask if you want to format your hard drive. If you answer N as in no, it might be formatted anyway if N has been programmed to mean Y.
It’s not really a virus, since it just uses a built-in feature of ANSI.SYS, but it can be used to make trojans still gets some virus-paranoid people going.