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A digital or electronic image is created by one of two different methods: raster (typically photographs) or vector (typically illustrations/drawings).
 Vector, or line-art,
artwork is created by graphics programs using mathematically specified
formulas and functions which create connected points on line and curve
segments. Because vector artwork is created mathematically it is easily
"scaled" or re-sized without loss of image quality. Each time the
image is sized up or down the formulas used to create it are
re-calculated. Vector artwork is the preferred method for screen printing, stamping and embossing. It can also be used in offset printing. "Drawing" or image creation
programs like Adobe Illustrator create vector or line-art artwork.
Raster, or bitmapped,
artwork is created by specifying or "mapping" the color tone or gray
level of each pixel individually within the "grid" or image area
(imagine a sheet of graph paper). The "grid" making up the  image
area is a specific height and width with a fixed number of pixels. The
number of individual pixels (lined up side by side) per inch within the
grid determines pixel size. These image types are
the best at producing gradations of color. Image editing
programs like Adobe PhotoShop generate raster or bitmapped artwork.
More pixels per inch = smaller pixel size = higher resolution and more image detail.
Less pixels per inch = larger pixel size = lower resolution and less image detail.
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JPEG's,
JPG's, or GIF's are low resolution file types used primarily for quick
downloading and display over the Internet, or other "screen only"
viewing. These files typically are no more than 72 ppi
(pixels-per-inch). Thus their pixel size is fairly large, images do not
have as much detail, and they are unacceptable for commercial printing.
Printing low resolution images results in an image that is blurry and
"jaggy" or "pixelated" (the image looks like it was made up of blocks like
a mosaic). See the example on the right.