There are 2 different types of images used by graphic design programs, raster images and vector images.
RASTER:
A raster image is made up of thousands of little dots/pixels, photo editors such as Adobe Photoshop are raster based and are great for rendering rich, full colour images like photographs. Raster based programs do have some drawbacks though:
==> Imagine a square 1 inch x 1 inch, if this square has been created at 300dpi then this will have 300 dots/pixels within it. The computer must keep track of all the zeros and ones that make up those 300 dots/pixels, this can result in large file sizes which can be memory intensive when editing, the spec of your PC/MAC will determine if this causes you problems or not.
==> Raster images do not resize well, when you resize a low resolution raster image the pixels just get larger making the image appear distorted and blurry. One solution to this is to ensure the image is created at high resolution, an image at a minimum of 300dpi will resize quite well and keep fairly good clarity, however, it will only enlarge so much.
VECTOR:
Vector based programs such as Adobe Illustrator approach image creation in an entirely different way and do not render images on a pixel by pixel basis. Using the same example as above the 1 inch x 1 inch square would only be made up of 4 dots/pixels, one on each corner. These “vector points” allow the computer to play connect the dots, each vector point has information telling the computer how to connect each point with straight or curved lines, and what colour the inner space should be.
Because the computer only has to keep four points in its memory, it is much easier for the computer to edit vector based images as file sizes are really small. If you resize a vector based image it loses little or no detail, the vector points spread out and the computer just redraws the image. Vector images are ideal for logos as they can be resized and adjusted without losing clarity, so when looking for a logo designer ensure the final files produced are vector based.
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explanation of Raster Images vs. Vector Graphics
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