What is the primary difference between a projection and a projected coordinate system?

The primary distinction between a projection and a projected coordinate system lies in the comprehensive nature of the latter. A projection refers specifically to the mathematical transformation applied to represent the curved surface of the Earth on a flat plane. This transformation allows features on the Earth's surface, such as points and lines, to be represented effectively on a two-dimensional map.

On the other hand, a projected coordinate system encompasses not just the projection itself but also includes defined parameters such as the coordinate units, standard parallels, and the origin point. It provides a framework for spatial data that is specifically tailored for a region and ensures that spatial relationships are maintained correctly within that context. This added layer of information is essential for effective mapping and analysis, making the projected coordinate system a more comprehensive concept than simply the projection alone. This is why the chosen answer accurately captures the essence of what differentiates the two.