There are three types of usersstandard, administrator, and System Administratorand 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.
References
The following Knowledge Base documents (located at www.apple.com/support) will provide you with further information regarding user accounts in Mac OS X.
Fast User Switching
25619, "Mac OS X 10.3, 10.4: Some applications only work in one account at a time"
User Access
93460, "iMovie: Using FileVault Can Affect Performance"
106156, "Mac OS X: Changing or resetting an account password"
106824, "Mac OS X: How to change user short name or home directory name"
107180, "Mac OS X: How to manage user access to applications, system preferences, and disc burning via 'Capabilities,' 'Limitations,' or 'Parental Controls'"
107297, "How to get files from a previous home directory after Archive and Install (Mac OS X)"
Open Firmware Password
106482, "Setting up Open Firmware Password Protection in Mac OS X 10.1 or later"