Digital Orthophotos

What is the difference between an aerial photograph and a digital orthophoto?

A conventional perspective aerial photograph contains image displacements caused by the tilting of the camera and terrain relief (topography). It does not have a uniform scale. You cannot measure distances on an aerial photograph like you can on a map. It is not a map.
The effects of tilt and relief are removed from the aerial photograph by the rectification process to create an orthophoto.
An orthophoto is a uniform-scale photograph. It is a photographic map.
Since an orthophoto has a uniform scale, it is possible to measure directly on it like other maps.
An orthophoto may serve as a base map onto which other map information can be overlaid.