Representing the three dimensional world onto a two dimensional piece of paper is achieved through the use of map projections. Projecting the curved surface of the earth onto a flat piece of paper or computer screen introduces spatial distortions.
There are many different types of map projections, with each projection distorting the earth’s surface in different ways. Unfortunately there is no one-size fits all, so cartographers must choose the best fit for the map’s purpose. Some projections are good for preserving area (such as Albert’s Equal Area Conic), some are better for preserving shape (Lambert’s Conformal Conic), while others are better for maintaining accurate direction (Mercator). The appropriate choice of projection depends on the scale, extents of the area to be mapped, the location on the earth, and the intended use of the map.
There is a good introduction to map projections on the ICSM web site.