The two main groups of image types are raster and vector. Raster images like JPEG, TIFF, GIF, BMP, and PNG are composed of pixels and can develop a “blocky” quality as the image is made larger because each pixel becomes larger. Vector images, like CGM and SVG, store lines instead of pixels. Using this method image size can be scaled without losing quality. The same vector image will appear correctly on a business card or a billboard.
Files with the JPEG extension are convenient for sending pictures over the internet due to their lossy compression. JPEG is a means of reordering the image in a way that after only a small part of the image has been downloaded, a hazy view of the entire image is presented rather than a crisp part or just a small part. The degree the compression takes can be adjusted to achieve the desired trade-off between file sizes and visual quality. JPEG images can also be stored with a JPG extension.
Compression to JPEG is used by quite some image file formats. Files with the JPEG extension (together with Exif) are the most common format for image files used by various digital cameras and other image or photo capturing devices. They are also quite commonly stored with the JPG extension in comparison to the JPEG extension.
Here's a small, but not exhaustive list of programs that can open JPEG documents: