What is the difference between iMac and Mac pro?

Yes, exactly as Kappy has stated.

The iMac is an "all-in-one" computer similar to an HP Envy, or HP Touchsmart.

HP Envy Here: http://tinyurl.com/bb-hp-envy

HP Touchsmart Here: http://tinyurl.com/bb-hp-touchsmart

These are an "all-in-one" machine, where the CPU (central processing unit), mainboard, and graphics display adapter are all embedded onto one board, and it is built-in to the monitor display case.

So you just have what looks like a "thicker" monitor on your desk and your keyboard connects to this "all-in-one" computer. (and actually the newest 2012 iMacs are not really that "thick" at all, and it does look pretty nice/thin/sleek).

The disadvantage to an "all-in-one" is that it's not easily upgradeable. You can't change out the graphics cards, or install new PCIe expansion cards (faster network cards, RAID adapter, etc.), can't add extra internal hard drives (i.e. 4 internal hard drives), etc.

The Mac Pro is a standalone computer. It requires an additional monitor/display. The advantage of having a Mac Pro is that it is upgradeable (you can get more powerful graphics cards in the future if you need better 3D rendering power) and it has far more memory slots, so it can be expandeable to I believe 128GB of RAM (whereas I believe the iMac might be 32GB of ram?)

iMac memory expansion here: http://www.apple.com/why-mac/compare/

Mac Pro Memory expansion here: http://eshop.macsales.com/shop/memory/Mac-Pro-Memory#1333-memory

Also the iMac is a much thinner device, and you can't separate the display from the CPU (they are an "all-in-one" device) so you can't upgrade or change the primary display, but you can add a second display).

With the Mac Pro, you can add additional graphics cards, and you can power up to twelve displays (if you wish). You also have a much more powerful processor on the Mac Pro (one quad-core, or up to two 6-Core Intel Xeon processors). So the Mac Pro a much more powerful machine.

The iMac is just a nice cute/thin desktop display (all-in-one), but with a quad-core processor it's still a fairly fast/decent machine.

If you are doing heavy 3D graphics rendering or gaming, or graphics design (3D CAD, etc.) then I would probably go with something more powerful like a Mac Pro, since you can get better graphics cards, and the Mac Pro will run much cooler (as you're trying to do massive rendering).

> Do you need an iMac to get a mac pro or does it come with a monitor?

No, you don't need an iMac for a Mac Pro (they are two separate and different computers), but yes you do need an external monitor display if you buy a Mac Pro.

Any 1920x1080p monitor/display with an HDMI input will be sufficient for using with a Mac Pro.

iMacs are All-in-One desktop models, Mac Pros are full-sized tower models. Mac Pros do not come with a monitor. They are completely separate and different.