All coordinates refer to a specific datum or projection. Before two coordinates can be compared they need to be in terms of the same system. The process to change coordinates between datums and projections is called either a conversion or a transformation.
Transforming coordinates is a multi-step process as described in the diagram below. Projection coordinates (metres North and East) must first be converted to datum coordinates (latitude and longitude) and then transformed back to the target datum and/or projection.