An orthophotograph has the spectral qualities of a photograph but the spatial attributes of a map. Orthophotographs therefore may look simply like an aerial photograph but directions, dimensions and plan positions can be all scaled from them, which makes them far more useful! One of the disadvantages with maps is that are a simplified representation of reality and have been derived for a particular purpose and scale. An orthophotograph contains all visible information without this simplifying abstracting process removing potentially useful information.
Orthophotographs are derived using using established photogrammetric methods where distortions dues to relief (i.e. topography) and tilt (i.e. camera wasn't perfectly vertical during original photo acquisition) in the original aerial photograph are removed.