There are three types of users—standard, administrator, and System Administrator—and each user has different capabilities.
Creating and managing users is done in Accounts preferences.
Security preferences provides options to increase user account security.
How users are able to log in, including enabling the fast user switching option, is managed in the Login Options pane of Accounts preferences. Other available options include whether or not a list of users is displayed in the login window, and which account, if any, the computer should automatically log into.
The following Knowledge Base documents (located at www.apple.com/support) will provide you with further information regarding user accounts in Mac OS X.