Windows 7 supports two kinds of users: local users and domain users.

A computer that is running Windows 7 has the ability to store its own user accounts database. The users stored at the local computer are known as local user accounts.

• Local user accounts are user accounts created in Windows 7. They exist only on the local computer.
• Local user accounts are stored in the Security Accounts Manager (SAM) database. Within the SAM database, each user account is assigned a Security Identifier (SID).
  
  Read more about Security Identifier (SID) at:
  http://support.microsoft.com/kb/243330.

Active Directory is a directory service that is available with the Windows Server 2008, Windows Server 2003, and Windows 2000 Server platforms. It stores information in a central database called Active Directory that allows users to have a single user account for the network. The users stored in Active Directory's central database are called domain user accounts.

If you use local user accounts, they must be configured on each computer that the user needs access to within the network. For this reason, domain user accounts are commonly used to manage users on any network with more than 10 users.

On Windows 7, Windows Server 2008, Windows Server 2003, Windows XP, and Windows Vista computers you can create local users through the Local Users and Groups item.

On Windows Server 2008, Windows Server 2003, and Windows 2000 Server domain controllers, you manage users with the Microsoft Active Directory Users and Computers utility.

Windows includes a command-line utility called Whoami (Who Am I?). You can use Whoami to find out the name of the account that's currently logged on, its SID, the names of the security groups of which it's a member, and its privileges.

• To use Whoami, open a Command Prompt window. (You don't need elevated privileges.) Then, to learn the name of the logged-on user, type whoami.
• If you're curious about your SID, type whoami /user.
• To see a list of your account's group memberships, type whoami /groups /fo list.
• To learn which privileges are enabled for the logged-on account, type whoami /priv /fo list.
• For a complete list of Whoami parameters, type whoami /?.