Is taliban shi'a or sunni? and why are they fighting us?

i asked a friend and he said US are fighting taliban because of 9/11. but they attacked 9/11 because they didn't like US's views or freedom. but then i asked why do they care what we viewed? so he said because US did something? and there wa a whole line of stuff i forgot what he said. then it somehow lead to why did Nato attack them or why did they attack NATO. so please tell me why this whole war started, if you know about the current politics and warfare