What are the main differences
Google the above suggestions I’ve pointed out and decide for yourself. Some are cromo, some ally. Most jump bikes have approximately a 14″ seat tube, 22-22.5″ effective Top Tube, are designed around 100mm forks and have nice short back ends. Though of course there are variations on the theme.
what should I look out for in kit or faults etc
Strength! Don’t try to get away with lightweight parts, its just not really worth it on a jump bike.
how do I get my jeans to hang halfway down my ass without them falling off?
I think you’re asking the wrong people. You’d be better off asking some 14 year olds down the local skate park. I think the total lack of any arse whatsoever is pretty vital though, as is the lack of any fashion sense!