You can think of crossover exactly like odin - it intercepts and translates win32 api calls into native os calls. Crossover is a commercial version of the Linux WINE project (they have a symbiotic relationship - codeweavers contributes a lot to the WINE project in general).
WINE is an open-source project released under the LGPL. It requires no windows license to operate (you
do still need a legal license of the software you want to run on top of it, however!). It is a clean-room implementation built using published microsoft API guides (none of the questionable code ownership or license tainting issues which haunted odin). They are not beholden to microsoft, but microsoft CAN break their product by introducing new api variants (not so much a problem for older apps, since win32 API has been stable for years, but newer apps can be a challenge)
More info on WINE here:
http://www.winehq.org/ (warning: heavy geek content)
I've got a few linux customers who purchased crossover simply so they could run Visio without having to boot windows. It works
extremely well for the short list of applications listed on their supported application list (
http://www.codeweavers.com/products/cxmac/supported_apps/). If your app isnt on the list, it may or may not function properly (unless you want to do a bunch of command-line jiggery pokery, and even then results are mixed)
I've used WINE on linux for years, with varying degrees of success. If you're a web developer working on a non-windows platform, it allows you to test your design on different versions of ie without rebooting (though you should be warned that there can be rendering differences between ie on windows and ie on crossover/WINE).
In my case, I'd rather use a virtualization product like parallels or vmware- first, it keeps windows in its own little sandbox where it cant do any damage to your host; second, you aren't dealing with emulation, you are running the actual application on the actual target platform - in my web example above, for instance, ie renders pages EXACTLY as the customer running windows does. It also allows for virtual machine snapshot support, which allows me to return a machine to a known state easily (no simple task on an actual windows box)
Crossover is great if you just want to run a handful of windows apps without leaving your nice osx environment. Don't asssume that software not on the compatibility list will work - odds are that it wont.
Parallels will have a mac pro version working soon, and will probably beat VMWare out the door with the product (and at less than 1/3 of the price of VMWare, to boot). $60 for crossover which can run a few applications but doesnt require a windows license, $80 for parallels that can run a number of virtualized operating systems but requires OS licenses.
Mac Pro Quad Xeon 2.66 Mac OS X (10.4.7) 
