- A projection is used to 'flatten' the ellipsoidal shape of the earth to rectangular coordinate system (i.e, to make a 'roundish' globe into a flat map).
- A datum is a specific, known location on or in the Earth that is used as a reference.
All GIS coordinate systems use a datum as a point of reference (i.e., it's location on Earth).
There are two types of "coordinate systems": Geographic Coordinate System (latitude and longitude) and Projected Coordinate System (X and Y).
A geographic coordinate system is not projected (not flat), they are in latitude and longitude. Think of a round globe (i.e., Google Earth), not a flat map.
Projected coordinate systems on the other hand are "flat" - but still need a point of reference (a datum) to define locations in space.
Both geographic coordinate systems and projected coordinate systems use a datum to reference the coordinate system to a location on Earth.