SUU (Server Update Utility) is a repository of updates for use with SBUU to update system firmware all at once from outside the OS (it can also be run from inside the OS to update drivers and firmware).
SBUU (Systems Build and Update Utility) is the utility used to assist with installing an OS or updating firmware. It does not contain OpenManage Server Administrator (OMSA) or drivers for installing 2008 and later OS's - the Driver Pack is necessary when installing these OS's using SBUU alone.
SMTD (Systems Management Tools and Documentation) is an all-in-one utility that contains SBUU, drivers for all supported OS's, OMSA install files, diagnostics, etc. If installing 2008 or later, SMTD is by far easier than installing with SBUU. So, it includes SBUU, but also all the supporting files for an install.