Originally Posted by GKM
I've not heard any Sigur Ross stuff but from the quick search I did it seems they are quite famous. That's gota be good for a relatively obscure style like ambient. I'm happy to call em ambient if it brings new listeners to the style.
Sigur Ros is normally categorized as "post rock".
As with all genres, the difference is fuzzy, but to me there is significant difference. Post rock is usually pop / rock first with some *influence* of non-rock genres in the textures used. Sigur Ros, like many other "post rock" bands, definitely are ambient influenced, however, they still utilize rock / pop hooks and structures quite frequently.
While music classified as "ambient" can be melodic as well, ambient music tends to be a lot more abstract at its core, with quite a bit of the genre as simply pure texture music with no melodic element or structure at all in many cases. The more an ambient piece moves away from structure, melody or rhythm, the more "niche" it becomes. ("Ambient music" can sometimes be popular to be honest if it has a touch of any of the above.)
"Post rock" also isn't necessarily ambient, you have bands like Godspeed You Black Emperor! in that category who aren't terribly ambient influenced at all.