Dirt jumping is one of the names given to the practice of riding bikes over cement type jumps of dirt or soil and becoming airborne. The idea is that after riding over the 'take off' the rider will become momentarily airborne, and aim to land on the 'landing'.
Type of bike
Dirt jumping can be done on almost anything with wheels, but it is usually executed on a bicycle.
BMX bike built for dirt jumping tends to have a longer top tube than a street BMX bike, and may well be more reinforced. They will rarely have pegs fitted (unless they are also used for street riding), and will generally run only a rear u-brake. Also, the tires will be treaded, as opposed to the slicks and semi-slicks used for park riding. Large, padded seats are also popular as something to land on when a trick fails, and are also easier to hold for in-flight tricks; but very small seats are also a style for the more "hardcore" riders. The gear ratio is generally around 44:16, though using small gearings like 25:9, known as 'micro gearing', has become popular in the present day.
Hybrid BMX/Jump bike - a scaled up BMX with 24" wheels. Strong alloy rims and lightweight chromoly frame. Suited to bigger jumps or more challenging competition courses.