A map is a 2d representation of the Earth’s surface. The latter is curved in a close approximation to a sphere, so a map projection is used to translate the Earth’s surface to a flat 2d plane. A datum is the approximation to the Earth’s surface that is used in this conversion. The coordinate system defines the coordinates used in projected space. As well as geographic longitude, latitude; a projected map is often used with a Cartesian (x,y) coordinate system.
These topics are described in more detail on their own pages: