You could see it as rows and columns, as Excel is built.
It is a useful when you want to structure things for easy access.
So for the 2D vector in the code below you have 100 rows and 100 columns.

So if you write: Value[1][10]
You are accessing the first row in the 10:th column.

Hope this helps to understand.

Remember, of course, that in C++ everything is zero-based. So Value[1][10] is the second row and the eleventh column.

A vector, as Jennifer84 demonstrates, is a superior construct, since you get all the benefits of an array without any of the drawbacks.

[EDIT] Oh yeah, almost forgot to answer your Q. A vector is a "container class" in the STL. Read what the C++ FAQ has to say about it here.