The key difference between pixel and vector based graphics is how the image is structured. Pixel based graphics are made up from lots of tiny physical squares (or ‘pixels’) where as vector graphics are mapped out using mathematical equations which calculate where the edges of the shapes sit in relation to one another.
As an example; fonts are always created using vectors so that the colours can be changed efficiently and the symbol can be easily scaled up to any size without losing quality. In contrast, digital photographs are always made up of pixels, which allows for a much more efficient blending of the colours as each physical pixel block can be coloured individually – rather than each shape being mapped out and filled with a gradient, as it would have to be if it were made up using vectors.
In a lot of cases, the format of a graphic can be identified quickly by it’s file extension – files ending in .AI or .EPS are vector based graphics, where as files ending in .JPG, .BMP or .PNG will always be pixel based graphics. There are some file extensions (such as .PDF) that overlap in this area and the quickest way to check without access to graphics software is to zoom into the image as far as you can – if the image doesn’t lose quality then it is almost certainly a vector based graphic.