.spec files are used in rebuilding RPM's. The proper command is rpmbuild <options> <specfilename.spec>. The .spec file tells the rpmbuild command how to configure and compile, as well as how to install the files later, a source tarfile before creating an RPM or .SRC.RPM from it.
1. Download the sources for the rpm you wish to build.
2. Download the .spec file.
3. Follow the instructions at: http://dag.wieers.com/howto/bits/rpm-build-user.php
4. Open a term and type "rpmbuild -ba packagename.spec" to build an .rpm and .src.rpm
There is more information about building rpm's here: http://fedoranews.org/tchung/rpmbuild/