keychain helps you to manage ssh keys in a convenient and secure manner. It acts as a frontend to ssh-agent, but allows you to easily have one long running ssh-agent process per system, rather than the norm of one ssh-agent per login session. This dramatically reduces the number of times you need to enter your passphrase.

the physical medium (such as a USB flash drive) used to hold the encryption key (and possibly the beginning of the boot process)

a very tough place to live images/icons/wink

a small chain, usually made from metal or plastic, that connects a small item to a keyring

A Mac OS X application that stores all of your usernames and passwords for easy retrieval by the keychain aware applications.