Can someone please explain autism spectrum disorder?

wow what i'm going to ask is going to sound kinda lousy but here goes. My brother was diagnosed a year and a half ago just before he turned 5 and i love him so much, don't get me wrong. Since his diagnosis he's improved so much it's amazing. HE started school and he can now count up to five which is a huge improvement. But i still don't really know what ASD is, i don't want to ask anyone in my family because i guess they just assume i know. If someone could just explain this to me i'd really appreciate it :)