Photogrammetry is the science of making measurements from photographs.

The output of photogrammetry is typically a map, drawing or a 3d model of some real-world object or scene. Many of the maps we use today are created with photogrammetry and photographs taken from aircraft.

Other definitions
Wikipedia Photogrammetry
Dictionary definitions at Answers.com

Types of Photogrammetry
Photogrammetry can be classified a number of ways but one standard method is to split the field based on camera location during photography. On this basis we have Aerial Photogrammetry, and Close-Range Photogrammetry.

In Aerial Photogrammetry the camera is mounted in an aircraft and is usually pointed vertically towards the ground. Multiple overlapping photos of the ground are taken as the aircraft flies along a flight path. These photos are processed in a stereo-plotter (an instrument that lets an operator see two photos at once in a stereo view). These photos are also used in automated processing for Digital Elevation Model (DEM) creation.

In Close-range Photogrammetry the camera is close to the subject and is typically hand held or on a tripod. Usually this type of photogrammetry work is non-topographic - that is the output is not topographic products like terrain models or topographic maps, but instead drawings and 3d models. Everyday cameras are used to model buildings, engineering structures, vehicles, forensic and accident scenes, film sets, etc.

See the links page for a list of resources and photogrammetry software.