Many people don't realize that there is a difference between not only the riding style of flatland bmx and street/park bmx, but also between the bikes. A flatland bike has a few very noticeable differences. For one, the handlebars have a greater length between the cross bar and the handles themselves. This allows for more room to jump between or stand on them. Another big difference is the length of the top tube. Flatland bikes also normally have a front brake and a back brake with a gyro to allow the handlebars to spin freely without getting tangled up. A shorter top tube on a flatland bikes allows the rider to manipulate the bike easier and do spin tricks easier. There are also some more subtle differences such as the space between the forks and the tire, allowing the rider to grab the fork easier for some tricks.
A street bike, on the other hand, has a frame that is normally longer. It can have pegs but they are often only on one side to save weight. Some riders will go brakeless for park and street, but it is smart to have a back brake on it.