| An interesting little tech that Apple came up with. The idea was to paper over the increasingly large number of different usernames and passwords that people are given.. well, practically everywhere these days-- by letting you just use one single password for everything. The keychain puts all of your passwords into a single secure systemwide database encrypted with one single password, allows apps to store and retrieve passwords when the user says they can, and gives users one single centralized place to check and remember their passwords. And you can have more than one person storing a keychain on a single machine without it affecting the others. The only downside is, of course, that while the technology itself is completely secure, if someone learns your Keychain password they can learn all your other passwords..
The keychain was originally a part of Apple's ill-fated PowerTalk technology (which died miserably, in part because other than the Keychain aspect of it no one was ever quite sure what it did or why they would want to use it..), but returned in Mac OS 8.5 and is now part of Mac OS X. Applications must specifically be written to support the Keychain, but adding keychain support is unbelivably easy. | 
