The internet started life as a text-only medium - the nearest anyone got to graphics was using emoticons! However, with the advent of the web, it was clear the Internet had a multimedia future - at least, people could exchange graphics and postage stamp-sized movies with each other.
As CPU power grew and bandwidth increased, the quality of media on the web has also improved - we've gone from strictly low-coloured images, to full-colour animations, fully realising the web's potential. This evolution of online graphics has led to a number of competing graphical standards, each with their own uses.
Today, the competing standards can be broadly divided into bitmap, where the colour of each pixel is stored in the file, and vector, where the mathematics required to draw the picture is stored (e.g.: circle of X radius centred at Y,Z co-ordinates). In between there is Adobe's Portable Document Format, which was designed to solve the "paperless office" problem.