Basically it's like... Well, imagine if a robber broke into your house. Would they go for the front door? Absolutely not. The back door? Of course. An example of a backdoor entrance is the following:
1. On the welcome screen of your computer (the screen where you select which account to log on to on the computer in XP/Vista), press Ctrl+Alt+Delete. If nothing happens, try it again. A different screen should come up.
2. On this different screen, if the password hasn't been changed, enter:
--Username: admin
--Password: password [OR] admin [if one doesn't work, try the other]
3. If the password has not been changed, you should now be logged in to the administrator account on the computer. You can edit settings for/delete/add accounts here. If you're smart though, you should change the admin password to something else, if it hasn't been changed already.