All mountain and cross country bikes are VERY different things...
XC bikes are essentially the road bikes of the mountain world. Lightweight builds and maximum pedaling efficiency are the big factors. Climbing is also much more prevelant in XC riding so the angles are geared to match that need. XC bikes will generaly be VERY upright, 71-72 degree head angles for quick handling and agility, 74 degree or so seat angles for good power transfer and, at the very most, 100mm or just shy of 4 inches of travel. Jumps and drops will snap XC frames. Often times the shocks will also have lockouts to further assist in power transfer on climbs and smooth flats.
All Mountain bikes are bikes which pedal well enough to handle XC rides but which have the angles and the strength to ride just about anything you'll come across. They're efficient pedalers but have more travel, usually 160mm or so, and are built to be great all around bikes. You can pedal it all day or take it to a bike park and hit most everything there but the really gnarly DH stuff. All Mountain bikes are perfect for people who dont want to have a 'stable' of bikes for their riding needs.
As an example, take Trek's lineup...
The Fuel is the XC platform; it has a 70 degree head angle, 73.5 seat angle, and 100mm travel front & rear. Those angles make it easy to pedal uphill and on flat but make it downright sketchy going back down or at high speed on rough terrain.
The Remedy is their All Mountain platform; it has a 68 degree head angle but still a relatively steep 73 degree seat tube angle, and 160mm travel front and rear. This means it'll still pedal very well with the steep seat angle, though you'll suffer a bit more climbing with the extra weight and travel... and can descend with just about anything but a DH bike with it's slacker 68 degree head angle and longer travel.
Modern All Mountain bikes are amazing in my opinion... If I had to own just one bike, it'd probably be the 2010 Giant ReignX0 or the new Santa Cruz Nomad carbon... an All Mountain bike like either of those and you can ride anything but DH and never worry.