Sometimes the software you want to install is not available in the Ubuntu repo's and when you find it on the web you discover that "oh noes, it's only available as a Red Hat "rpm"
Never fear, it is usually possible to convert the rpm to a deb using alien. This should be done as a last resort however, it's always better to use a native deb package wherever possible.
The alien package is available from the multiverse repository. If you need to enable this repo see this post here
sudo apt-get install alien
To convert an rpm;
alien --to-deb --scripts package-xyz.rpm
As always, you should change package-xyz to the name of your package,