BMX stands for "Bicycle Motocross" and is an offroad bicycle sport including bike racing and bike stunts. It comes in a variety of bike styles including: BMX race, dirt, park, street, flatland and BMX freestyle. BMX bike wheels come in sizes 16", 18", 20", 24", 26" to 29", with 20" being the most popular size. BMX started in the early 1970's on the dirt tracks in Southern California. The most well known BMX makers are: Redline, Haro, Free Agent, WeThePeople, Se Bikes, GT and Norco. Important for the street credibility of an BMX brand is foremost it's BMX team. Haro, Free Agent, GT and Redline all have the best leading BMX teams, but the newer brands are stepping up and receive a lot of attention from the local riders.
BMX sizing chart here: