What are the licence terms for a werdpress theme taken form a wordpress.org website?

I was just wondering what are the licence terms on those types of wordpress themes.
Under what licence are they distributed? Can they be used for commercial purposes? Basically, my question is – if a theme is free, what am I allowed and not allowed to do with it?

I would like to know as much as possible

Thanks.
Ok, Can a product/software under "GNU General Public License" be used for commercial purposes?