Shanon is right.
typically, in a small - medium environment (up to 25000 seats) you will install ACCCM on a single server (either psychical or virtual). in this case you will install all of the platform components on a single server:
DB (sql 2005/2008 express/STD/ENT) *note that by default ACCCM wizard install the 2005 express edition - no license required
Application server (hosted on windows server 2003 x32 or windows server 2008 x32 /x64)
Web server (Microsoft IIS 6/7)
if you want to go HA you can distribute the different system components across 2-8 servers (depended on the HA requirements)