What is District 13?

I haven't read the books. I hoped the movie would have been sufficient to fill me on the details presented in the books as it did pretty efficiently in the previous 2 movies.

So what exactly is a District 13? Since each District has a particular purpose for the Capitol, like District 12 was coal miners, other districts were wood cutters, farmers & so on. So did District 13 manufacture & supplied weaponry to the Capitol? If not, how did they get their hands on such advanced weaponry?

Also, District 13 is built underground. Is it built on the original site of District 13 after the Capitol bombed them or is it located in some discreet location?