So I understand the difference between the sports but how diifferent are the bikes? are they essentialy interchangeable or are there special racing BMXs?
BMX bikes are all very similar. Short, stout frames, tall handlebars, 20" wheels (and maybe the occasional 24" for a very tall BMX rider), and wide tires. The difference between Supercross and Freestyle BMX bikes is in the gearing. Supercross bikes will have a higher gear ratio for getting up to speed in races, while Freestyle bikes usually have a lower gear ratio to make pedaling easier at low speeds. Some freestyle bikes will come equipped with brake cable gyroscopes, to allow for barspins, but they aren't as common now as they were during the height of BMX's popularity.