Mac Pro is a high quality ("pro") computer. Mac OS is operating system software for Mac computers. Hardware versus software. Apple is the only company that make both computers and operating systems.
OS X is the "new" type of Mac OS. The Macintosh computer was announced in 1984, using an operating system simply called "System 1.0" and evolved into "Mac OS 7.6" and the Arabic-only numbers continued until OS 9.2.2, the last of the "Classic Mac OS". Apple took a big chance by changing the basics of the operating system with OS 10 (or "X" in Roman numeral). The X also fits with NeXTSTEP, the operating system that was the ancestor of OS X, and with UNIX, the operating system that forms the raw, basic power and security under both NeXTSTEP and OS X. Note: Linux copies many features and arrangements of UNIX but is not certified as UNIX. Windows is based on the features and arrangements of DOS with an appearance that copies Mac OS.
My simple reduction of choices: Windows to be like the vast majority, Linux to be a maverick geek, OS X to get things done. The differences are much smaller than these simple categories suggest, but you often need to compile applications using programming code in Linux before you can get started using those applications. It is a golden time for geeks and a waste of time for me. Most serious professional creators want to keep far away from viruses, so they opt for Mac unless they need to use some specialized necessary software that is only available for Windows. Virus infections rob you of time and time = $.
Mac Pro is the new Intel processor version of what was called "PowerMac" from 1994 to early 2006. The two basic choices are based on number of processors, and after that the power of processors. Since the first PowerMacs, Apple has designed their workstation computers for using multiple processors. There was even a Mac clone computer in 1996 that came with four separate processors. Nowadays, processors can have multiple cores inside each one, so a single processor acts like two or four processors. You can opt for as much as two quad-core processors in one computer (almost the same as having eight processors). The numbers don't tell the whole story. The type of processor is also important. The best type now is called Xeon "Nehalem". The advantage of processor speed (some number of gigaherts or GHz), processor type, and number of processors is related to how much data can be used for some purpose in a given time span. Always think about the future. What we ask a computer to do in 2009 is much more than what we expected it to do in 1999.
After choosing the processing power, you choose:
* the size and number of hard drives (up to 4 internally),
* the amount of RAM (up to 32GB), and
* the type of video card.
Larger capacity drives means you can store more and larger files on the hard drive(s).
RAM is electronic storage space -- it goes off and is lost when the computer shuts down. It's where your files are shown in front of you before they are saved to a hard drive. Save the files or loose them. More RAM means the computer can open more applications one after the other without closing any, and it can do more with those applications without slowing down.
Video is the most complicated detail. You will need to ask the experts if you want to run games. There are different aspects to games that require different ability from various video cards, so opinions vary. For video editing, any card that Apple sells with a MacPro will be stupendous.
You don't need to think much about expansion ports on a Mac Pro since it has four FW800, and five USB 2.0 ports, plus two more USB 2.0 ports on the keyboard. Theoretically, you could attach eleven external devices at once without adding any expansion cards.
An important aspect of video editing is rendering (making computer files out of raw data). For that, the data path to the storage (typically a hard drive) is very important. Internal drives are the fastest. Next are external Firewire drives. Don't let anyone fool you into believing a USB 2.0 connection is faster than the old FW 400. In actual use, the FW400 is much faster, damn the numbers. FW800 is superb for video rendering. All Mac Pro computers have four FW800 ports.
In the box:
* Mac Pro
* Power cord
* Apple Keyboard with Numeric Keypad
* Apple Mouse
* USB keyboard extension cable
* Install/restore DVDs
* Printed and electronic documentation