Distributed Computing, also known as "Grid Computing" is where you install a program onto your computer that works in the backround, and uses your spare CPU resources to do various forms of scientific research, depending on what client you are using. Some examples of these programs are Folding@home and World Community Grid
Below are some tutorials to help you install and configure these types of programs.
FoldingAtHome - Several methods on how to setup Folding@home
WorldCommunityGridTeamUbuntu - Tutorial on how to set up BOINC, a free open source program that runs many different kinds of DistributedComputing programs, including World Community Grid. Also has information on how to "crunch", or contribute work towards Ubuntu's team on World Community Grid, Team Ubuntu.