Caps lock is a feature nearly all keyboards have that, when active, capitalizes each letter typed on the keyboard. Unlike most keys on the keyboard, the caps lock key is a "toggle key," which is always in either an on or off position. When off, the letters typed are input like usual. When on, each letter typed is transmitted as an upper-case character. Caps lock does not alter the input from the rest of the keys, such as numbers and symbols.
Because caps lock changes the input of the letters on the keyboard, it is important to know whether it is on or off. Therefore, most keyboards have a light on the caps lock key that turns on when active, indicating caps lock is on. Because passwords are often case-sensitive, it is smart to make sure caps lock is not active when entering login information. Also, it is best not to use caps lock when typing e-mails because IT MAKES IT SEEM LIKE YOU ARE YELLING when you type in all caps.