When you first setup Mac OS X it has you create an account. By default this account has administrator rights. An Administrator account in OS X has rights to: create other user accounts, install software in the Applications and Library folders and change any computer settings.

Depending on how much you need to do these things you may not want your primary account on your OS X computer to have these rights. When you create other accounts on the computer you can chose to make the account an administrator or regular user. A regular user would not be able to install software on the computer and would not be able to change many of the settings.

If you are working in a regular user account it is harder to accidentally install malicious software or change settings that could compromise the security and stability of the computer. If you know the administrator account login credentials you can still install software and change setting, but you must provide those administrator credentials when you go to do so. Thus you gain certain security benefits by working in a regular account, while still being able to administer the computer without having to log out of one account and into another.

To create a new account open System Preferences under the Apple menu at the top left corner of the screen. Then select Accounts under the System section.

Next, if necessary, click on the padlock toward the bottom of the window and enter your administrator account credentials. Then click the "+" button, right above the padlock, to create a new user account.

Enter the name of the account, a short name (these can be the same), a password, and verify the password. Optionally you can also add a password hint. Then if you want the account to be a regular account leave the checkbox for "Allow this account to administer this computer" unchecked. If you want the account to be an administrator check the box. The administrator access can be changed later if desired. Click Create Account to finish creating the account.