elpenguino:

 

Speaking of which, I see bikes marketed as enduro, cross country etc. What are the major differences?

 

XC (Cross Country) - generally short-travel (sub-110mm) and often a hardtail (no rear suspension) but there are some XC full-sus bikes around now - steeper headtube angle and lower front-end (generally built like a racing bike with flat bars). Often ridden by people in lycra trying to get their Strava time up (more expensive XC bikes), or entry-level XC bikes being sold to new riders and the general public. 

Downcountry - generally 120-130mm travel forks and sometimes rear suspension as well. Set up in-between a XC and Trail bike. Super-light, short-travel and slightly more aggressive geometry than XC (slightly slacker HT angle - maybe 67-68deg)

Trail - slacker and more aggressive again - modern Trail bikes generally have 130-150mm travel front (and most often) rear. Slightly beefier than XC and DC bikes. HT angle of around 65-66deg these days. Made predominantly as "do-it-all" bikes for the average to aggressive rider. Also found in Hardtail as well as Full-Suspension. Lots of gear range for climbing to the top of whatever you might throw yourself down. 1x at the front and 11 or 12 speed 11-52 at the rear isn't uncommon. 

Enduro - long travel trail bikes that go up hill OK as well as being insanely capable downhill. Long, low, slack. Tough, long-travel suspension (usually 150-180mm travel front and rear) and in some cases long travel front-suspension with a hardtail and super-slack geometry. Also have capability of many DH bikes, but with less weight and wider range gears. Seen often at Bike Parks heading downhill at speed and defying gravity over anything that resembles a jump. 

Downhill (DH) - specifically made to point down a hill and go as fast as possible. 200mm + travel front and rear usually. Often heavy with beefy forks (38/40mm diameter stanchions) and usually dual-crown. These are the weapon of choice for adrenaline junkies and 16year-olds who like to break limbs. Often seen being pushed up the slightest gradient, or attached to a ski lift or trailer to be taken to the top of a mountain for a rapid descent. Tackle rock gardens, jumps, cliffs, drop-offs and other scary stuff with ease. 

Then of course there are Dirt-Jumpers, Gravel Bikes, weird hybrids of all of the above and whatever niche a specific manufacturer might want to cook up (google "Grim Donut"). 

All of this assumes the "right now" of mountain biking. A couple of years ago, Downcountry didn't exist. Only a few years ago, head angles and seat tube angles were different and everything I said above wouldn't have been true. In a couple of years, this will probably all look like nonsense as well. An Enduro bike from this year, with the most modern gear and geo, could easily be considered a DH rig from 5 or 10 years ago.