How many different versions of Windows Server 2003 are there?

Windows Server 2003 Web Server
This edition supports only up to 2GB Ram and is restricted for use in a webhosting environment

Windows Server 2003 Standard Server
This edition is for small and medium businesses. This edition replaced Windows 2000 Server

Windows Server 2003 Enterprise Server
This edition is for medium and large businesses. This edition replaced Windows 2000 Advanced Server

Windows Server 2003 Datacenter Server
This edition is aimed for high end servers and supports up to 64 processors. This edition replaced Windows 2000 Datacenter Server

Windows Server 2003 Compute Cluster Edition
A edition of Windows Server 2003 for clusters and supercomputers

Windows Small Business Server 2003
SBS 2003 includes Windows Server 2003 and a few other Windows Server System products. SBS is aimed for small businesses and have (license) restriction over the other Windows Server 2003 products