Young Trumpeter

i hear teachers of mine mention (often complaining about) modern or contemporary classical/concert band music (not rock or rap or that kind of contemporary). what, and when, is the difference between modern and not-modern music?

also, what exactly is the difference between concert band, symphonic band, wind ensemble, wind symphomy, and any others i missed? different groups in high school and around where i live are all called these different names but is there a difference in size, instrumentation, music selections, etc?

Thanks,
Aaron