Georeferencing is a method of overlaying aerial imagery on top of already made maps. The HaPET office I was working for used these maps to follow wetland easements. Aerial imagery was taken every year as separate pictures and need to be matched up to the terrain. This was done in arcMap. They have a georeferencing tool that allows you to pick points on the aerial image and pick a matching point on the known map allowing you to precisely line up the image.

This is a very powerful way of monitoring wetland drainage and to look for changes in land cover. Here is a written tutorial on the process.