Mountain biking used to be a single sport with a single type of bike. Over the years it has evolved into a number of separate discliplines:
Downhill bikes are 'finely tuned race machines' (according to Adam from Orange) or 'built like great ugly tanks' (according to any roadie). They are incredibly robust with long travel suspension to take the knocks and rigours of getting from top to bottom as fast as possible with health and sanity reasonably intact. Examples are the 224 from Orange and the Giant Glory.
Freeride is a bit of downhill, a bit of cross country, and a bit of everything else inbetween. Freeride bikes (such as the Giant Reign X and the Orange Big-T) are built to take the bumps and abuse that result from carrying a rider that isn't prepared to let terrain get in the way of progress. Typically they have slightly shorter travel suspension than downhill bikes and are a bit lighter.
All-Mountain bikes such as the Orange Patriot and the Giant Reign are probably the most versatile of the mountain bikes. They're generalists rather than specialists, capable of operating well on a range of different terrain types. Essentially a compromise between the light weight of a cross country bike and the robustness of a freeride bike.
Cross Country bikes such as the Giant Anthem and the Orange P7 are the lightest of the mountain bikes. Designed to cope with flatter terrain than the other breeds of mountain bikes they are fast and responsive. Where suspension is used, it is short-travel to maximise response and efficiency. Cross country bikes are also noticably faster uphill than their heavier (and bouncier) cousins.