Unlike traditional image raster files, which are defined by a series of pixels, vector files store visual information as a series of mathematical instructions on x- and y-axes positions. Consequently, vector images can be reproduced at whatever size you wish without any deterioration in quality. In addition, because the files are smaller in size, they are ideal for use on websites.
Raster images can be stored in many different formats, but their most important attribute is their resolution, or, to put it another way, their level of detail – measured in dots per inch (dpi). The higher the resolution, the greater number of pixels and so the better the quality. This is important when it comes to enlarging something like a logo or graphic, as you will find that unless you have a very high resolution, the quality of the image will significantly deteriorate as you increase the size – resulting in an unprofessional pixellation of the image. When you have an image in vector graphic either in eps, ai or pdf format, you have the ability to use it as a vector graphic in print or online. Or if you need to use the image in a raster format such as jpg, you still have the ability to save the image at whatever size and resolution (dpi) you want. Either way you get an image with high resolution and at a best possible quality.