JPEG --
Joint Photographic Experts Group -- JPEG is best for photos when you need to keep the file size small. JPEG is not suitable for images with text, large blocks of color, or simple shapes, because solid lines will blur. Best used on the web.
GIF -- Graphical Interchange Format -- GIF files can be animated, where the others can't. Best used on the web.
PNG -- Portable Network Graphic -- PNG are useful when you need smaller file sizes with no loss in quality. Best used on the web.
BMP -- Bit Map Picture -- Use BMP for any type of bitmap (pixel-based) images. BMPs are huge files, but there is no loss in quality.
PSD -- Photoshop Document --