Cross Country Mountain Bikes
These bikes, the most traditional, are designed for general trail use and are lightweight, allowing for more nimble climbing and maneuvering. The bikes contain a full selection of gears, generally 24 or 27, to allow for the most fluid, efficient pedaling. Cross country mountain bikes can be full-suspension (front and rear), hardtail (front only) or rigid (no suspension at all). Their suspension systems feature less travel than other bikes, generally up to 4 or 5 inches.
Race mountain bikes are basically high-performance cross country bikes. The main characteristics, including suspension and drivetrain, are comparable to regular cross country bikes, but these bikes are designed to be lighter and faster. They are among the most expensive bikes on the market and the extra money purchases the lightest possible high-performance components demanded by pros and racers. These bikes are created to be fast and efficient.
This discipline involves shuttling up the mountain and racing down at high speeds while chewing up obstacles. Freeride, which originated later, involves hitting huge jumps and features, taking monstrous drops and throwing down tricks.
Bikes are often designed to encompass both disciplines, though some bikes are specifically for one or the other. Since both disciplines concentrate on downhills and speed, qualities such as light weight and full drivetrain are less important. These bikes are the heaviest on the market, designed for durability and speed. They feature full-suspension with the greatest amounts of travel, typically 7 to 10 inches in back, to eat up the large amounts of vertical resulting from high jumps and drops. They don't contain a full selection of gears like mountain bikes and have only one or two front gears with a full cogset in back.
In diversifying the gear choices, many companies have created a separate category of mountain bikes between downhill/freeride and cross country. These bikes are heavier and sturdier than typical cross country bikes and feature a suspension with more travel, in the 5- to 7-inch range. However, they incorporate a full drivetrain (i.e. 24 or 27 gears) and are designed to be used for full trail use. Essentially, they're a great option for cross country riders that like to push it on downhills, jumps and obstacles.
Dirt Jump Bikes
Dirt jump bikes are a cross between a mountain bike and BMX bike, designed for dirt jump parks and similar riding. They're smaller and lower than any other mountain bike and feature a drastically-angled top tube. They come in hardtail and rigid designs. They feature a limited selection of gears and are sometimes even single-speed. Some dirt jump bikes have 24-inch wheels, smaller than other mountain bikes which feature 26- or 29-inch wheels.
Most major mountain bike companies offer women-specific bicycles. These bicycles feature altered geometry, more accurately fit the bicycle to a woman's body. The top tube is shorter allowing for more comfortable positioning.