A File Extension is a group of letters displayed after a file name, showing the purpose of the file.
Portable Network Graphics (.PNG) FilesEdit
Portable Network Graphics (PNG) is a little bit like the best of both worlds. It was developed based on GIF, for lossless compression and for display of images on the web. Unlike GIF, PNG supports 24 bit images and produces background transparency without jagged edges. Some older web browsers, however, do not support PNG images. PNG format supports RGB, Indexed Color, Grayscale, and Bitmap mode images. PNG also preserves transparency in grayscale and RGB images.
Graphic Interchange Format (.GIF) FilesEdit
Graphics Interchange Format (GIF) is one of the file formats used to display indexed-color graphics and images in HTML documents on the web. Indexed color means that it will only display a maximum of 256 colors. For this reason, GIF is NOT a good format for saving photographic type images with many colors. GIF is good for saving images with flat blocks of color such as logos or simple illustrations. Another very important feature of GIF images is that it allows you to preserve transparency. It uses an LZW-compressed format designed to minimize file size.
Joint Photographic Expert Group (.JPEG) FilesEdit
Joint Photographic Expert Group (JPEG or JPG) format is used to display photographs and other continuous-tone images on the web. JPEG format supports CMYK, RGB (millions of colors), and Grayscale color modes. Unlike GIF format, JPEG retains all color information in an RGB image, but compresses file size by selectively discarding data. This is known as lossy compression, and can result in a loss of quality if a high level of compression is applied. In most image editing programs you can specify how much compression/loss of quality you want. Generally, if you choose the maximum quality option, your image will be indistinguishable from your original photograph. However, you can also save your files with lower quality settings that will still produce a reasonably good image (or at least good enough for the web) image. Another important difference between GIF and JPG, is that JPG does not preserve transparency.