RPM stands for RedHat Package Management and was developed by RedHat Linux as a way to simplify the management of software on a Linux box. RPMs are software packages rolled in with information about the software it includes. In SUSE this infomation is used by YaST
to keep track of what software is installed, what version, where, and more. RPMs exist for SUSE, Debian, RedHat, and many other popular Linux distributions.
RPMs can best be explained with an example. Imagine wanting to install an MP3 player on your SUSE box. You could use the regular "install" version and install it, but if you ever wanted to uninstall it you'd need to go back and delete all the files created all over your hard drive. This can be very confusing when you consider you have countless other software packages installed. Finding dependencies and installing updates can cause hassles, especially for new Linux users.
So RPMs are created by Linux community members for countless versions of Linux for countless software packages. These RPMs are built specifically for certain distros and hardware configurations. For example, a RedHat Thunderbird RPM for i686 architecture will likely not work on a SUSE box with an i386 processor; the individual would need to locate the right RPM for them. The filenames of RPMs are standardized and significant, the naming convention is explained here.
My favorite sources for RPMs are RPM Search, rpmseek.com, and PackMan.