DPI (dots per inch, or pixels per inch) is a measurement of the output quality of an image, and refers to the resolution of images on a screen or printed page. The more dots, the better the resolution. Pixels are the picture elements which make up an image, similar to grains in a photograph or dots in a half-tone. Each pixel can represent a number of different shades or colors, depending on how much storage space is allocated for it. Sometimes the preferred term is pixels per inch
(ppi), as it more accurately describes the digital image.
JPEG is the abbreviation of Joint Photographic Experts Group file format, which is a compression “recipe” for condensing the size of image files. JPEGs are helpful in allowing access to full-screen image files on-line because they require less storage and are therefore quicker to download into a web page. This is generally the best format for photographic images being used on the Web.
TIFF stands for Tagged Image File Format, typically used for saving files to be used in print applications because they are uncondensed and lose no quality in the file-saving process, unlike JPEGs.
GIF is Graphic Interchange Format file format, typically used for saving files to be used on Web pages because it employs an efficient compression method and allows the image to be represented with a transparent background.
