For questions specific to the "Opera Mini" web browser. See the 'browser' tag wiki for a list of related tags, including those for other browsers.
Primarily designed for mobile devices, the Opera Mini webbrowser can reduce your web traffic by directing it via Opera's own proxies for compression.
Redirecting all web traffic via Opera's own proxy servers, Opera Mini can help you to save parts of your data plan. This is mainly achieved by compressing images. While this may speed up page transfer (and reduce traffic, as said), it comes at a price of image quality -- and it won't work in "closed environments" (e.g. accessing local resources which are out-of-reach for Opera's proxy servers).
Opera Mini 7.5 (Source: Wikipedia)