JPG (or .JPEG) is a raster image that is often used for photographs on the web. .JPGs can be optimized, when saving them out of Photoshop, to find the perfect balance of small file size and high quality. On the web, you want your images to be as small as they can be so your site loads quickly, but large enough to still appear crisp and not pixilated. A JPG can’t have a transparent background so they are always in the shape of a rectangle or square with a solid background.
Best use = rectangle or square photos and photographs on your website.
PNG is another raster image type. For the general marketer, the main difference to understand between a .PNG and .JPG is that a .PNG can have a transparent background and is generally larger and higher quality. Therefore a .PNG is ideal for saving logo files for websites because they can be placed over a coloured background.
Best use = Logos, icons and other images where a transparent background is preferred.
GIF is another raster image type. A GIF is formed from up to 256 colors from the RBG colorspace. The fewer colours and shades contained in an image, the smaller the file size. Therefore a GIF is ideal for images that use just a few solid colors and don’t have gradients or natural shades. In other words you wouldn’t want to use a GIF for a photograph.
Best use = simple web graphics such as web buttons, charts and icons.
TIF(or TIFF) is a large raster file. It has no loss in quality and therefore is primarily used for images used in printing. On the web, because of load time, you generally want to use smaller images such as JPG or PNG.
Best use = Images and photographs for high quality print.
EPS file is a vector file of a graphic, text or illustration. Because it is vector is can easily be resized to any size it needs to be. An EPS file can be reopened and edited.
Best use = Master logo files and graphics and print designs.
AI file is a proprietary, vector file type created by Adobe that can only be created or edited with Adobe Illustrator. It is most commonly used for creating logos, illustrations and print layouts.
Best use = creating logos, graphics, illustrations.