Keychains are for keys, but what exactly does that mean? The digital data you can store securely in a keychain falls into several major categories.
Without a doubt, the vast majority of items you store in your keychain are passwords of one kind or another. Each password in a keychain also includes the associated username (if any) and the location of the item — usually a URL or an IP address. A keychain may contain passwords such as the ones needed for the following:
Local servers, including other Macs on your network using file sharing, screen sharing, and other shared services
Remote file servers, including FTP servers and WebDAV servers, among many others
Websites (nearly any website that requires you to log in with a username and password)
AirPort base stations and Time Capsule devices
Applications that use encryption or that must access password-protected network services on your behalf (including chat, blogging, synchronization, and telephony applications, among others)
Encrypted disk images
Although passwords are fairly self-explanatory, some of the other objects stored in keychains may be less well-understood. The next set of items has to do with verifying someone's identity in a different manner than simply asking for a password.
Public key cryptography is a clever way of encrypting data without the sender and recipient ever having ...