Well, here's a better way to put it.

Garnet VM is a Virtual Machine, or an emulator, of the Palm OS for Maemo. It can run on top of Maemo seeing as it's an emulator.

Android, as stated above, is a full-blown OS by Google that can either run along-side (or on top of) Maemo or on it's own. Most users like running it along-side since they just want to use it instead of really debugging it. Those that like to debug or are programmers, usually flash it (if that's possible now) to their device for testing, debugging, programming, etc.

Hope that helps to clear things up a bit!

Jesse~