In the video below Oracle RAC or Oracle Real Application Cluster is explained in less than 3 minutes. It will not go over all the nice a good topics however in a very very short time you get the general idea. If you like the idea you can go into more detail on the Oracle website or even build your own Oracle RAC test environment in a virtual environment and play around.
If you like to know more about building a test environment and even a production environment with a combination of Oracle RAC and Oracle VM you can have a look at the whitepaper "Oracle RAC in Oracle VM Environments".
If you want to start building your test environment you can have a look at "Build Your Own Oracle Extended RAC Cluster on Oracle VM and Oracle Enterprise Linux".
A good example of a situation where you would like to use Oracle RAC can be found in this previous post.