Many Adobe software products including Photoshop CS include an activation system to ensure they are used according to the license agreement. Authorization codes based on a serial number associate your copy of Adobe Photoshop CS to your computer.
After a copy of Adobe Photoshop CS is installed on a computer, it needs to be activated to work properly. The serial number and information about the computer is sent to Adobe, and then authorization to use the software is sent back to the computer. Typically, if a serial number has already been used on one computer, it won't work again on another computer unless the software is removed and deactivated from the first machine.