All mice have at least one button, though most mice have two or three. Some also have additional buttons on the sides, which can be assigned to different commands. Most mice also have a scroll-wheel, which lets you scroll up and down documents and Web pages by just rolling the wheel with your index finger.
Early mice tracked movement using a ball in the bottom of the mouse. This "mouse ball" pushed against different rollers as it moved, measuring the mouse's speed and direction. However, now most mice use optical technology, which uses a beam of light to track the mouse's motion. Optical mice are more accurate than roller-based mice and they have the added bonus of not getting dirty inside.
A plurality of plurals
When you refer to more than one mouse, you can call them either "mice" or "mouses," since both terms are acceptable. However, "mouses" is technically the correct version since "mice" is the plural form designated for living creatures. Still, most people have a hard time saying "mouses," which is why "mice" is more commonly used.