bzip
bzip is a compression program. It is rarely used today, having been replaced with bzip2, which offers much better compression.
The bzip kioslave is not directly usable, and is intended for use as a filter. For example, the tar kioslave can filter a file through the bzip kioslave, in order to display the contents of a tar.bz file directly in a window.
If you click on a file compressed with a bz extension in , this kioslave is used to uncompress it and display it as a normal (uncompressed) file.
If you are a developer, and would like to use the bzip filter, you can find documentation on using kioslaves at http://developer.kde.org