SCUMM = Script Creation Utility for Maniac Mansion
Maniac Mansion has been the first graphic adventure in the Lucasfilm Game / LucasArts style (you now, point and click adventures).
As the name says, SCUMM is a utility to CREATE games like Maniac Mansion, Indiana Jones, Day of The Tentacle, etc.
(BTW It's not available, before you ask
).
Using a SCUMM interpreter, computers can play these scripted games. When you buy a LucasArts adventure, the appropriate EXE file is the interpreter.
To play it on other systems like GP32 or Dreamcast (or older games on newer PCs), you need an interpreter. As LucasArts never wrote one, fans created SCUMMVM - a freeware interpreter for these games.
So SCUMMVM is NOT an emulator, it's an interpreter for Lucasfilm / LucasArts games.