Nuke,
rpms (or debs, or whatever) are the binnaries, that is, the equivalent to the .exe and .com files under Win and DOS.
.tar.gz are the equivalent to .zip, that is, compressed files, which usually, when talking about an application, contain the source code of that application, not the binnaries. So, after compiling those files in the -tar-gz file, you get the binnaries and you can run the application ;-)
If you already have the rpm file, you can run the application with no prblems. If you want to compile your own app with your own tweaks, get the .tar.gz file and cimpile the sources.
Hope that helped clering concepts :-)
Regards