Mac OS X 10.6: About keychains
You can use keychains to reduce the number of passwords you have to keep track of. A keychain can store all your passwords for applications, servers, and websites; cryptographic keys, or even sensitive information unrelated to your computer, such as credit card numbers or personal identification numbers (PINs) for bank accounts.
When you connect to a network server, open an email account, or access any password-protected item that works with keychains, your keychain can provide the password so you don’t have to type it.
You start with a single keychain named “login” which is your default keychain, and is created automatically the first time you log in to your Mac OS X user account and has the same password as your account. This keychain is unlocked automatically when you log in to your account.
You can create new keychains to store passwords for different purposes (for example, one for work and one for online shopping), or you can make a copy of a keychain to take with you to other computers.
Keychains can be accessible to just a single user, or shared with the other users of the computer.