A look at Keychain access (and why you should care)
Here's a conundrum: last time you switched on your Mac OS X computer, did you have to enter a password before it loaded up your user account and desktop? No? Funny, because the computer asked for one. You didn't see that part, nor did you see your computer provide a password for itself--but that is, in effect, what happened. Your computer comes with a neat collection of security features built-in, but if you have it set up to automatically log you in every time you boot it up, you're bypassing one of the most basic of them.