What is the difference between an Intel i7 and a Xeon quad core processor?

I know the Xeon processors have been around for a long time and are mostly used in servers, but I am curious, why do people not use the Xeons in a high performance desktop?

As far as I know about the best desktop processor out there now is the Intel Core i7 line. The i7's and Xeons are both quad-core processors, what is the main difference in these? I just saw that the MacBook Pro's are using the quad core Xeons instead of the i7's.

The current line of Xeons are based on the same architecture as the i7. The difference is usually that the Xeons are the cream of the crop. They run cooler and at lower voltages and are spec'd for 24/7 continuous usage. Otherwise, performance is usually identical. Xeons are able to be used in multi-socket motherboards, where i7s are not (which is why the Mac Pros use them).

Xeons are also usually the first to be updated. There are 6-core Xeons, but not i7s yet, though they are still based on the same architecture.

The additional reliability of the Xeons is very important in servers, especially rackmount and blade-enclosed servers where the lower heat dissipation and power consumption are essential. These benefits usually don't mean too much to all but the most extreme overclockers though, so people in the desktop segment usually ignore it due to the enormous difference in the price of both the CPU and the motherboards needed to support them.

Another difference between Xeon and i7 is that the Xeon supports ECC memory, the i7 does not.

Also, some Xeons are designed to work in multi-CPU systems, whereas absolutely no i7 models do. As such, if you want a multi-CPU system, you must use nothing less than a Xeon.

why do people not use the Xeons in a high performance desktop?

They do, but they're referred to as "Workstations".

Workstations are essentially a server, but with expansion abilities more like a desktop.

You get all the benefits of a server (symmetric multiprocessing, stability, management, hot-swap, etc.) plus the things you wouldn't normally have in a server, like video card(s).

Check out Intel's Workstation pages to find out about their current Workstation boards like the S5000XVN, S5520SC, and WX58BP.

We've been building systems like these (SMP Xeon workstations) for years for AutoCAD usage at many clients, and for various number crunching machines at the local university (engineering, stats, astronomy, etc.).

They're awesome to use, and expensive. :)

http://ark.intel.com/ This is the intel product comparison site. You can look through and compare the different intel processor lines and see many of the design and feature differences.

In addition to what has been mentioned: Some of the Xeon's also support a couple advanced processor features that the desktop iX processors do not. Most of these however are only relevant and useful in scientific and advanced computing needed in workstations, massive multi-processor machines, and CPU intensive applications.

Advanced power management and thermal properties are essential to high performance computing and servers. In this market, electricity and cooling are significant costs. Using a certain processor that is designed for the server environment might save a company lots of $$$ each month in power bills if it can process much more efficiently. This is just one example.

Also not previously mentioned in the other answers,

Much larger caches on Xeon where the number of cores are roughly equal. Although the L3 shared cache on the top of the line 10 core xeon is a whopping 30MB.

Higher core count on Xeon. Currently 6 cores on i7 vs 10 cores on Xeon.

And some extra processor instructions on Xeon's such as Intel Trusted Execution Tech and hardware AES etc. You generally wouldn't see these features used on a desktop machine so you can see Xeon's are geared towards servers or machines with special requirements.