Passwords should be at least 6 characters long with no spaces. They should contain only alpha numeric characters (a-z and 0-9). For added security it is a good idea if at least one of the characters is a number.
Passwords ARE case sensitive, so remember the case you've used.
No passwords are stored on the website - instead an encrypted 'footprint' of the password is saved, and used for comparision when you log in. If you forget your password you can reset it using a link sent to you by email, which is why it is important that you add your email address when setting an admin account up.