mod_gzip is an Internet Content Acceleration module for the popular Apache Web Server. It compresses the contents of your web pages delivered to the client

The mod_deflate module allows output from your server to be compressed before being sent to the client over the network.  This enables the automatic GZIP-encoding of all MIME-types, except image and PDF files, as they leave the server.  Image files and PDF files are excluded as they are already in a highly compressed format.  By compressing your files with mod_deflate you save save money by having less bandwidth transmissions.