Browsers that support the favicon's are Internet Explore, Mozilla Firefox, Opera, etc. The most commonly encountered favicon sizes are: 16x16, 32x32, 48x48, 64x64, 128x128. A good favicon can improve the image and design of a website. It can be logo, or anything derived from a site or forum topic. You must be careful that when you choose a favicon image it must be as .ico format to be accepted by all browsers. Generally a favicon is loaded favicon in the root domain. To achieve a favicon use a specialized graphics program.
You must use a favicon considering the fact that most peoples bookmarks / favorites list is a very cluttered place. Having yours stand out with a nice graphical reminder of your site beside it is a good way to get their attention again.