The Mercator and Gall-Peters projections are two projections on the extremes of the spectrum, ranging from angle-correct to surface-correct. Mercator has historically been used to navigate the seas, because if you draw a straight line on it, it's actually straight, with angles preserved. Sadly it got adopted as the 'standard' map projection, with large distortions at the poles (look at the South Pole, appearing to be larger than any continent, which obviousely isn't true). The Gall-Peters on the other hand wants to display surface area correctly, and heavily deforms topology (the borders of countries, the way they're connected), stretching them until they fit. The problem with it that I have is that the 'Peters supporters' said that the Mercator was partly responsible for colonialism, and that their map is 'much better', which is absolute nonsense. It didn't cause colonialism and no map projection is 'the best', as they're all compromises from going from a 3d to a 2d surface.