The [tex]\in[/tex] symbol

I'm reading my calc chapters and I don't know what this sign means. I thought I post it in the pre-algebra & algebra section because the symbol has nothing to do with calc.

So I was wondering what the symbol is called and what it means. Here it is: $\displaystyle \in$

It is used in context like this: The Cartesian product $\displaystyle \Re x \Re x \Re = { (x, y, z) | z, y, z \in \Re } $ is the set of all ordered triples of real numbers and is denoted by $\displaystyle \Re^3$