Caps lock is a key most people never used except by accident. The caps lock key allows you to be able to type in all caps
Pressing the caps lock key activates it and does not require holding it down. Once you press it then all the characters you type will be as capitals as in THIS IS CAPS. Normally you do not want to type in all caps
If you turn it on then either pressing the caps lock key or shift key will turn it off depending on the system.
Typing messages in all caps in emails is considered shouting and is harder to read so try to avoid it.
Interesting that one computer manufacturer last week announced plans to drop the caps lock key. It will not be that different as most have not even noticed it is gone on our PDAs such as Blackberry