As adamk75 said. The RPM packages are for distributions based on Red Hat or OpenSuSE. The RPM acronym itself stats for Red-Hat Package Manager. Ubuntu is based on Debian and uses DEB packages. apt-get
is the right tool from the command line, synaptic
is a package manager with an easy Graphical User Interface.
There are rare cases when you have to truly install a RPM on a Debian based system. In that case you need to convert the RPM to DEB using alien
. Believe me, these are very uncommon situations.
Especially in case of development packages, like xorg-x11-devel, you must install the software specifically built for your distribution, hence
sudo apt-get install xorg-dev
was the right thing to do. Indeed, ubuntu provides the rpm package manager you've installed, but I think it's based on alien as well.
Lastly, here are some useful links:
Edit: I've just seen your latest post. Well done!!