If you're talking MOSS 2007, Microsoft lays out the differences here.
Unfortunately, the format sucks. Why not just a chart?
office.microsoft.com/.../FX101758691033.aspx

This guy does the summary for you:
blogs.officezealot.com/.../20032.aspx

I guess that first link is a chart. The CSS wasn't coming through the
first time I looked at it.

Yeah, I was talking about the Moss2007, and I gone through those two links
already

my concern is the sites  which are running  successfully on enterprise  can be
migrated to lower licenced version  of MOSS!! i.e to Standard licene or NOT?

did anybody done this type of migration/downgrading before !?

the only difference I Can see is "Business data search" .. !!
(as we haven't implemented the concepts "Business Process and Forms & Business
Intelligence ")

will there be any further loosing of functinality in the sites?

is it involved with any risks as the sites are bundled with lots of data and
very imp to organization!?

yeah, I can understand that the lower version sucks,but its up to the
organization to decide which head-ache verison they want to choose

Any Suggestions are welcome!

You can physically migrate  MOSS2007 enterprise sites  to Standard
edition. But you'll need to make sure that you don't use any MOSS2007
enterprise features. They will probably still work since the DVD is the
same for both standard  and enterprise, but you would not be licensed  to
use those features even if they work. The main enterprise  features are
Business Data Catalog, InfoPath Forms Server, Excel Calculation Server,
and some of the more advanced Business Intelligence features.

The Enterprise features can be enabled or disabled in a site/site
collection with a setting in the Site Features screen. I think you
should be OK if you make sure that is disabled before exporting the
sites.

Otherwise, you may get errors on the import where the manifest has
references to features that aren't available (even if they weren't
being used).