Here are IMO a set of definitions for bike classes, examples and applications.
XC: This is a bike built for speed over moderate terrain. The bike is usually a hardtail of FS up to 4.5 inches of travel. They will usually weigh less than 25lbs have steep head tube angles (70-72 in a 26 inch form). This bike accelerates and turns very quickly and will be a race oriented bike. They will also have a more flat back riding position, rougher ride and be a little twitchy on technical decents. A few examples include: Specialized Epic, Elsworth truth, Giant Anthem, Niner Air 9 Carbon.
All Mountain: This bike is made to handle very technical obstacles and park type environments, but is not quite a full on downhill bike as it will still go uphill. Most of these bikes are of the 26 inch variety and will have 6 to 7 inches of travel, 2.35-2.5" tires and rugged components. These will have head tube angles of 66-67 degrees and weigh 30-35lbs depending on build. Their agility in the buff comes at the price of slow climbing and turning response. A few examples include: Santa Cruz Nomad, Specialized Enduro, Pivot Firebird and Ibis Mojo HD
Trail: This is the Swiss Army Knife of mountain biking and probably the most popular. Suspension travel is usually 5-6 inches for 26ers, most FS 29ers, and some hardtails will hit this catagory as well. Head tube angles will be 67 to 70 degrees, and bikes will wiegh aprox 24 to 30lbs. These bikes will have a bit more relaxed riding position than the xc type and will be at home on all but the most technical trails. They will handle the rough with more confidence than xc's and some will be nearly as fast. 75% of the bikes you see at our local trails will be of this type and are the logical choice for most people who can have only one bike. Examples include: Specialized Stumpjumper FSR, Giant Trance, Elsworth Epiphany, Ibis Mojo, Trek (Fisher) Rumblefish, Pivot 5.7 and the list is endless.
Within each group you will find a vast selection of different pricepoints, suspension designs, component mixes and wheel sizes. Take an honest look at how and where you will be riding 90% of the time and try as many bikes in that catagory that your budget will allow.