The Mac Pro is a workstation class Mac with the ability to add more hard drives, graphics cards, optical drives, PCIe cards, upgrade the RAM easily in more than two DIMM slots and have choice of display.
The iMac is an all-in-one Mac desktop class with a built-in display, only one hard drive and graphics card and only two DIMM slots.
youll know when you need a Mac Pro and iMac's getting quad core wont diminish the sales of quad core Mac Pros.