i have no idea what these electronica genres mean most of the time. my own explanations for how i categorize things--

idm: does it sound like something the folks at warp records or the like do (e.g., squarepusher, aphex twin, boards of canada)

ambient: no drums

experimental: "harder" ambient with no discernable time signature

mellow: slower than idm, usually sorta melodic or with a traditional harmonic structure

i don't use the other genres much.