The planet Earth may be round, but it is far from a perfect sphere. Because the Earth spins on an axis, centrifugal force actually bulges the center (equator) outwards. Gravity also plays a role as it is not constant across the entire Earth's surface, and many of the instruments that measure the Earth use gravity as a part of the measurement. These are some of the factors in geodesy. Geodesy is the science of accurately measuring and understanding Earth's geometric shape, orientation in space, and gravity field.
In the following two sections, you will get a brief overview of the pieces that make up your map projection. There is a lot of complicated math involved in these pieces, and to the regular GIS user there is little need to know the math; but if you find yourself doing more research work, it is highly recommended you research the math involved.