What is this kind of layout called?

What is this kind of layout called?

I was hoping someone could tell what this kind of layoutting is called ?
See link
I mean, the way that the boxes are arranging itself beautifully, and no
excess space. I've even seen websites where they rearrange if one of the
boxes is enlarged or something.
So, how is this done ? Is it just pure CSS, via floats or something, or is
javascript etc. involved ?
Thanks in advance.