When you first boot a new Mac and set up a user, the system is configured to automatically log in that user. That’s probably fine if you’re the only person who uses that computer, but not so great if your Mac is sitting out where a lot of people have access to it. You’ll want to customize your security settings to fit the environment where you’ll be using your Mac. If it’s a desktop machine and you’ll be using it only at home, for example, you probably don’t have much to worry about. But if it’s a MacBook that you plan on hauling everywhere you go, you’ll want a little more security.
Security in OS X usually comes down to passwords: passwords for services, accounts, websites, and email. Once you’ve created all those pesky but necessary passwords, you’ll want to turn your attention to managing them.
To manage all your passwords, Mountain Lion uses keychains; they’re where it stores your passwords and certificates to keep them safe from prying eyes. These keychains save you a lot of time, because your Mac can use the stored passwords to do a variety of useful things, like joining your wireless network without any help from you.
The more you do online, the more passwords you need. Ideally, you want different passwords for everything; using the same password for your bank’s website ...