Joined: Aug 2008
Posts: 13,140
Likes: 2,163
From: Ann Arbor, MI
Bikes: 15 Kinesis Racelight 4S, 76 Motebecane Gran Jubilée, 17 Dedacciai Gladiatore2, 12 Breezer Venturi, 09 Dahon Mariner, 12 Mercier Nano, 95 DeKerf Team SL, 19 Tern Rally, 21 Breezer Doppler Cafe+, 19 T-Lab X3, 91 Serotta CII, 23 3T Strada
As Dan515 mentioned in the first reply to this thread, the lower gearing of MTB gruppos is designed for lower speed climbing over uneven and loose terrain, while a road bike gruppo is generally higher geared, designed for a narrower range of circumstances as encountered over paved roads on narrow tires.
Additionally, road gruppos are generally lighter and focused on precision shifting, whereas MTB gruppos are designed with crisp shifting as well, but additional focus on withstanding the dirty, physically abusive world of off-road riding.
Of course, the shifters are different between MTB and Road bikes generally, to accommodate the different shapes of handlebars and conform to custom, and the brakes different to accommodate wheels and frame mounting options.
It's more typical to find triple chainrings up front on MTBs along with long cage rear derailleurs to accommodate lower gearing. Road bike gruppos often emphasize aerodynamic styling and higher levels of detail finishing, while MTB gruppos often go for a more rugged look.
In practical terms, there's not a lot of distinction for most cyclists; one can ride Road gruppos on the dirt and MTB gruppos on the road, it just depends on needs. If you've got a lot of hills, need to pull trailers, or have weak legs, those things might recommend using an MTB gruppo (or road touring gruppo); if your terrain is flat, you're focused on speed, or want the lightest bike possible, those may be things to suggest a Road gruppo is the way to go.
In sum, I don't think you need to be worrying about the type of gruppo at all. You don't have the experience or focus to make any of the distinctions meaningful, and the bike designers have already made the important decisions for you with the bike spec. Just focus on the total package, and get the bike most fully satisfies your needs and desires. If you're riding road, get a road bike. If you're riding trails, get an MTB. If you're cruising around town just to get some fresh air and a little exercise, get a comfort bike. If you want to do long distance touring, get a touring bike.
Stick to the macro questions, and you'll be alright.