Bitmaps (pictures made of dots) can not be enlarged without badly degrading the quality of the images. All the TV crime shows with 'image enhancement' that show a clear pic of a mans face created from a video surveilance image of 6 pixels are rubbish! Vector images are able to be resized as large or small as you want and will not lose quality as they are not a record of pixels and colours, but rather a mathematical way of describing an image.
Cheers
Ross