HTTP Compression
To improve transfer speed and optimize bandwidth utilization, an HTTP server may compress data sent in response to an HTTP request. This is called HTTP compression.
The client and the server negotiate compression automatically. A client that supports HTTP compression will indicate its support in the request. Consequently the server will know when to compress the response files and what compression algorithm to use.
For example, a client that supports HTTP compression often sends the following request header:
Accept-Encoding: deflate, gzip
If the server supports HTTP compression at all, invariably it will support gzip and will respond with the following response header:
Content-Encoding: gzip
In fact, Apache's mod_deflate supports only gzip.
A historical anecdote