Explains it nicely. I can not improve on it.
In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Yep, another nice page.
It might be worth mentioning that an "outward facing" square bracket is sometimes used instead of a round one for open intervals.
eg. (0,1) = ]0,1[
() is my preferred notation though.
Why did the vector cross the road?
It wanted to be normal.