If you have a website or plan to have one soon, this quick little primer on web graphics format will help you understand how the most-used graphics formats differ from one another and, more importantly, when to use which type:

JPG, PNG, or GIF – When and How to Use Different Web Graphic Formats

As a commenter notes further down the page, there is a reason that GIF format is still in heavy use, despite the primer’s advice: background transparency. Essentially, saving a graphic in GIF format allows you to place the graphic on any kind of web page background and have that background show through. For more of an explanation, visit this brief GIF Transparency page.