GNU ar is a utility that archives a series of files. It is used to take a group of files, and combine them all into one. This allows one to then compress, send, or most importantly and commonly, link the files into a program.
The tool itself is about as simple as its purpose. To archive a series of object files:
ar -rcs archive.a obj_fil1.o obj_fil2.o obj_fil3.o obj_fil4.o ... obj_filN.o
Then, to link against them:
cc example.c archive.a -o example.bin
Important: If you link other object files besides the library archive.a, take care that the archive.a is right behind these object files (or have it be the very last file in the list of files to link) - so the linker can resolve the one or other occurrence of a symbol which is covered by some member of the archive.a library.