An eye opening material from iStockPhoto, on What vector illustrations are? & How to use them? If you are a newbie in digital illustration and using royalty free illustration, then this article is exactly for you. 

"First, a little technical information. There are basically two kinds of digital still images: raster (also known as bitmap) and vector. Raster images are made up of pixels, tiny colored squares arranged in a rectangular grid, forming an image. Vector graphics, on the other hand, use mathematical calculations to plot points — and paths connecting them — to describe the image. The resulting shapes, or objects, can be easily manipulated, colored and re-sized without loss of quality. Whereas a raster image may have, say, 1895 pixels from point A to point B, a vector graphic simply plots the two points, calculates the distance between them and draws a line (for this reason, vector files are typically much smaller than bitmap images)." Continue Reading