Software packages come in several flavours: one is RPM (Red Hat Packagemanager) from Red Hat, probably the largest North
American Linux "vendor". This flavour is also used by many other distributors/vendors.

Debian is another distributor, but uses its own package system (dpkg/apt) which in turn does not use *.rpm files but *.deb
files. Knoppixis derived from Debian and uses *.deb files. rpm and deb are not compatible, but that does not matter,
because all of the software can be installed in the respective native format. Well, almost...

Very rarely one might want to install software and only have an rpm. Here's where "alien" comes in: alien converts between
different package formats, in this case rpm -> deb. However, I would advise a newbie to not use this tool: there are reasons
why the two formats are incompatible.