Sets of files, on every Linux based device, come in "packages". There are various systems used, e.g. rpm on RedHat/Fedora, deb on Ubuntu/Debian etc. Embedded systems like set-top-boxes mostly use a system called "ipkg" (with filenames ending in .ipk). So ipk is what we're using. Kernel modules, tools and also plugins are all supplied into packages, not the other way around. Every plugin is supplied in a package. But tools or other stuff can very well be supplied in a package that has no plugin in it. Sometimes tools are installed because some plugins declare a dependency on them and then it may look like the plugin supplies the tool, but it really doesn't.
The only thing is that plugin packages can be installed from within enigma, so to install other, non-plugin packages, you need to do it from the shell, using the "opkg" command.