What is a member server?

A member server is any installation of Windows 2000 server (or NT, 2003 etc) that is in a domain but not a domain controller.

So just about any server in a domain that doesn't handle user authentication etc.

What Is the Difference Between a Domain Controller and a Server?
A member server:
Does not receive a copy of the domain account database, but does have access to it if it is a member of the domain.
Maintains its own local user account database.
Does not process domain logons.
Has the same built-in user groups as a Windows NT Workstation (including Power Users).
Can be configured as a member of a domain or workgroup.
MORE INFORMATION
You may have to make a determination about the role of a server in the domain. Use one of the following methods to determine the computer's role.
Method 1
Click Start, and then click Programs.
Click Command Prompt.
At the command prompt, type net accounts, and then press ENTER.
A list of information appears, and the last line of the information indicates the computer role.

Sample of the output:

c:\>NET ACCOUNTS
Force user logoff how long after time expires?: Never
Minimum password age (days): 0
Maximum password age (days): 42
Minimum password length: 0
Length of password history maintained: None
Lockout threshold: Never
Lockout duration (minutes): 30
Lockout observation window (minutes): 30
Computer role: PRIMARY
The command completed successfully.

Method 2
Right-click Network Neighborhood.
Click Properties.
Click the Identification tab.
Click Change.
If you can change the computer to a workgroup, the server is a member server. You cannot do this if the server is a domain controller.
Method 3
Click Start, click Programs, and then click Administrative Tools.
Click Server Manager.
View the role of the computer in Server Manager.