What is a keychain?
You can use a keychain to keep track of passwords for applications, servers, and websites; cryptographic keys and certificates; or even sensitive information unrelated to your computer, such as credit card numbers or personal identification numbers (PINs) for bank accounts. Your default keychain is where new items are stored. Your login keychain is your default keychain unless you have configured it differently. Certain types of certificates may go in the System keychain.