Georeferencing refers to the process of taking an image of a map (in a raster format such as JPEG or TIFF) that you've scanned from a paper map, or a scanned image that you've found on the web, and adding locational information to it so that GIS programs or other mapping applications will be able to place it correctly onto the landscape.
Once properly georeferenced, you can overlay other layers on it or use it as the basis for creating your own editable layers (feature- or vector-based layers).
Georeferencing can be done fairly easily in GIS programs such as ArcGIS or QGIS, but the online apps below provide another alternative.
