Geocoding is the process of finding associated geographic coordinates (often expressed as latitude and longitude) from other geographic data, such as street addresses, or zip codes (postal codes). With geographic coordinates the features can be mapped and entered into Geographic Information Systems, or the coordinates can be embedded into media such as digital photographs via geotagging.
Reverse geocoding is the opposite: finding an associated textual location such as a street address, from geographic coordinates.
A geocoder is a piece of software or a (web) service that helps in this process.
http://en.wikipedia.org/wiki/Geocoding
http://googlemapsapi.blogspot.com/2006/06/geocoding-at-last.html