Here’s what I have found out – it’s a progression.
Basic Mountain Bike – Hybrid
– Not much more than a road bike with knobbly tyres, and maybe a token front fork suspension. Ok for off-tarmac, Fire Road, Canal Path.
XC Mountain Bike
– 100mm Travel, Light weight, Disc Brakes, Hard Tail mostly, some Full Sus. Built for off-road, low gradient, but not heavily technical terrain. Built for all round fast speed, especially up hill.
Trail Mountain Bike
– 120-140mm Travel, heavier & stronger than XC, Hard Tail and Full Sus. Built for more technical off-road, OK for moderate drops and jumps, moderate gradients, with a slacker geometry. Still relatively light. Fast to Moderate uphill speed.
All Mountain Bike
– 140-180mm Travel, heavier and stronger than a Trail Bike. Almost exclusively Full Sus. Built-in capability for most technical terrain excepting hardcore downhill. OK for larger drops and jumps, faster downhill capability, increased weight limits uphill speed and capability.
Down Hill Bike
– 160-200mm travel Iron Horse. Very heavy, super strong bike built for steep gradient heavy terrain, high drops and jumps at speed. Not intended for even moderate uphill, weight severely limits uphill capability (other than gravity assisted). Ultimate downhill capability and speed.
Then apply these various sub-categories which can be subdivisions of the above:-
Play Bike
4X
Dirt Bike
Enduro
Marathon
29er
Of course feel free to pish on everything I’ve stated above.