To answer your question basically: The processors found in the Mac Pro are faster than the ones in the iMac.

More Detail:
"Xeon" and "Core i7" are specific sub-brands of intel cpus that include many different processors with different speeds. "Nehalem" refers to the architecture of a broad variety of CPU's. All Core i3, Core i5, Core i7, 3400-series Xeon, 3500-series Xeon, and 5500-series Xeon processors use the Nehalem Architecture.

Another important fact is that "Xeon" processors are designed and optimized for use in servers, while "Core i7" processors are designed and optimized for normal users.

"Xeon" processors also have several more advanced design features to help them run faster than the "Core i7" processors, but "Xeon" processors also cost a lot more.

(The processors in the mac pro are the Xeon w3580, w3540, w3520, w5570, w5550, or E5520 The processor in the iMac is the core i7 860)

http://www.cpubenchmark.net/high_end_cpus.html

(Cpu bench marks are way down at the bottom, not all of the cpus are there.)

http://service.futuremark.com/hardware/

More info:

http://en.wikipedia.org/wiki/Nehalem_%28microarchitecture%29

http://en.wikipedia.org/wiki/Intel_Core_i7

http://en.wikipedia.org/wiki/Xeon

http://www.apple.com/macpro/specs.html

http://www.apple.com/imac/specs.html