Hi Tony,
On a broader scale the difference is the extend of various features that are available for use when you deploy one feature set IOS over the other. Enterpise incorporates many more features across many more platforms as compared to the Desktop feature set.
To find out the exact difference, you will need to know what platform is this question relevent to, which IOS version, in the minimum.
IF you go through the table in the release notes of any IOS version, you will see what I'm talking about. Below is a release note link for 12.3 IOS.
See if you can find the info you are looking for.
http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps5187/prod_release_notes_list.html
Thanks,
yatin