[c] What is a file with extension .a?

I downloaded this: https://github.com/mongodb/mongo-c-driver

And now I'm trying to use it inside my C program, but I don't know what to do with the generated .a files. What are they? I couldn't find any information, not even in the GCC manual.

And I built it like so:

  scons --c99

Also, can I use C99 libraries with my C89 program?

.a files are created with the ar utility, and they are libraries. To use it with gcc, collect all .a files in a lib/ folder and then link with -L lib/ and -l<name of specific library>.

Collection of all .a files into lib/ is optional. Doing so makes for better looking directories with nice separation of code and libraries, IMHO.