Curved sections of the Earth's surface are generally transformed through a projection system to representations on a flat map, and each map feature is located in a geo-referenced coordinate system with defined origin and units. There are many systems for projecting a spherical surface or portion of it onto a flat plane. Actual projection systems are based on various trade-offs between accuracies of position, shape and size . Equal-area projections preserve correct areas.
Conformal projections preserve correct shape.
Azimuthal projections preserve correct direction or orientation of all features relative to the center of the map. To understand some basic projection methods, think of the Earth as a transparent globe lit from a point in its center or a filament stretched between its poles. A cylindrical projection would result from wrapping a cylinder of paper tangent to the equator, and recording the projection from a filament between the poles. A conic projection would result from placing a paper cone over one hemisphere, tangent to some latitude, and recording the projection from a point light source at the center of the globe. An azimuthal projection results from placing a flat piece of paper tangent to a single point on the globe.