The MMC is where SP is launched from, where you record and modify all your scripts, and run your tests from. Agents are additional machines that may or may not have the MMC portion of the software installed. You can have 10 machines with the full SP installation and they will all 'work' as MMC 'controllers'. You could then run a test from any of them and use any of the others as 'agents' for where the actual virtual users are executed from. I'm taking a guess here that 'MMC' stand for Master Machine Controller', but don't quote me on that.