This was taken from an article I found online and you can read the full write out here.
"Windows Server 2003 Web Edition supports a maximum of 2 processors with support for a maximum of 2GB of RAM. Additionally, Windows Server 2003, Web Edition cannot act as a domain controller. It is the only edition of Windows Server 2003 that does not require Client Access Licenses when used as the internet facing server front end for Internet Information Services and Windows Server Update Services. When using it for storage or as a backend with another remote server as the frontend, CALs may still be required."