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.
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.