A keychain is an encrypted container that holds passwords for multiple applications and secure services. Apple Inc. uses keychains as password management system in Mac OS and iOS.
Keychain is Apple Inc.'s password management system in Mac OS and iOS. It was introduced with Mac OS 8.6, and has been included in all subsequent versions of Mac OS, including Mac OS X. A Keychain can contain various types of data: passwords (for Websites, FTP servers, SSH accounts, network shares, wireless networks, groupware applications, encrypted disk images), private keys, certificates, and secure notes.
In Mac OS X, keychain files are stored in ~/Library/Keychains/, /Library/Keychains/, and /Network/Library/Keychains/, and the Keychain Access GUI application is located in the Utilities folder in the Applications folder. It is free, open source software released under the terms of the APSL. The command line equivalent of Keychain Access is /usr/bin/security. The keychain file(s) stores a variety of data fields including a title, URL, notes and password. Other than Secure Notes created with Keychain Access, only the password is encrypted, with Triple DES. The contents of Secure Notes are also encrypted.
Source: Keychain
