sorry for this eternal question...

I'm simply doing an oral presentation in my ?english pop culture class? on trip-hop... My head is gonna explose with all these ?different styles? and ?evolution? of trip-hop. I've listen to Portishead, Massive, Tricky, Lamb, Sneaker Pimps... That's what I thought trip-hop was: Either samples with scratching with a light rapped voice or a usually soft, air-fluffed, female's voice melody. I don't know much on the subject but when I hear Tortoise or some of Jay Jay Johnson's stuff, it's an other direction... What is the difference between trip-hop, dub, downtempo, ambiance and etheral? Why are Bjiork, Thievery corporation, Smith and Mighty, Massive attack and Portishead all in one group? When I hear Nitin Sawhney I think trip-hop but I could do parralels with english light pop music like Dido...This shouldn't be good! Help me please with all these small perticular details...(sorry for my english I'm from Quebec ;)