As far as Active Directory is concerned, computers are very similar to users. In fact, computer objects inherit directly from the user object class, which is used to represent user accounts. That means computer objects have all of the attributes of user objects and then some. Computers need to be represented in Active Directory for many of the same reasons users do, including the need to access resources securely, utilize GPOs, and have permissions granted or restricted on them.
To participate in a domain, computers need a secure channel to a domain controller. A secure channel is an authenticated connection that can transmit encrypted data. To set up the secure channel, a computer has to present a password to a domain controller. The domain controller then verifies that password against the password stored in Active Directory with the computer's account. Without the computer object, and subsequently, the password stored with it, there would be no way for the domain controller to verify a computer is what it claims to be.
The Anatomy of a Computer
The default location for computer objects in a domain is the cn=Computers container located directly off the domain root. You can, however, create computer objects anywhere in a domain. And in Windows Server 2003, you can modify the default location for computer objects as described in Recipe 8.12. Table 8-1 contains a list of some of the interesting attributes that are available on computer objects.