correct me if I am wrong but this is where you create displacement maps of a hi poly model and use them to recreate the shape in a much lower poly model?
Retopology is the creation of a a new mesh over an existing mesh usually in the case of reworking serious topology problems in a mesh, or creating a mesh for game or animation over a mesh that is not optimized for such usage
So then would the term retopology be sort of general in that there are a number of ways to do it? Or is a projection method most common?
yes, new geometry snaps to the surface in projection like method or hand drawn lines that will be calculated for intersection, these are the mass used methods, i'm not aware of any other retopo methods