Copy and Paste Extreme is an application for sharing your clipboard between multiple machines.
Let's say you're in your office with a couple of Macs. You have some source code you want to send to someone in an email, but your source code is on one Mac, and your email application is on another Mac. You want to copy the source code to the clipboard on one Mac, and paste it into an email on the other Mac. This is what Copy and Paste Extreme does.