Most programs do come in .tar.gz and like you said, it is like zip. So it doesn't necessarily have to be a program in there. And since this is a Zydas 1211 usb wlan "driver" this could be the case for such modules or wrappers.
Suppose you have a .tar.gz file. Issuing the following command removes and decompresses the .gz portion:
$ gunzip filename.tar.gz
Resulting in a .tar file, use the following command to extract it:
$ tar -xvf filename.tar
Alternatively, use "zxvf" parameters to essentially to both commands.
$ tar -zxvf
Package unzipped, there are more powerful uses of tar:
$ man tar
Yes, README and INSTALL are important. Most programs will take:
# make install
At which point, you can ask:
$ which "program-name"
And you will get back the path to the program, which you can use to create a new icon on your Kmenu. Some will do it for you, others won't.
YaST will show you RPMs but not source files. Also look into apt4rpm, cool tool!