mod_gzip is a standard Apache Web Server module which acts as an
Internet Content Accelerator. Its function in life is to become an
integral 'part' of any existing Apache Web Server and see that the
content being delivered to the end-user, is as small and as optimized
as possible.

It makes use of the Accept-Encoding feature in most modern browsers
to deliver gzipped content transparently, and so cuts down on
bandwidth usage.