Depends on what type of riding you want to do. For BMX-type stunts - get a BMX.For going some serious distances both on and off-road - get a MTB.Don't really know what you mean by jump bike.
It all depends on what type of riding you do. If you do cross country, the best bike for that is probably some Cannondale, downhill/freeride would be a kona, orange or a giant. What is an "extreme...
Well, there is a look to them. 20" wheels, long, low frames. Handlebars look a certain way. Do a net search for bmx pics and you'll soon figure out what to look for.