MapQuest.com generates maps and directions on the fly. In the Web site's first month of existence, it got a million hits, and its instant success spawned an industry. Online mapping applications are now a dime a dozen, but MapQuest is still the top performer.

How does MapQuest.com determine the "ideal" route? What are the most common errors and why do they happen? In this article, we'll look behind the scenes of MapQuest.com to find out how it delivers maps and directions, and we'll check out some of its lesser-known applications.

MapQuest is pretty much your quintessential online mapping program. Its main functions are FindIt, which lets you find businesses in a particular area; Maps, which creates a location map based on address, city, zip code or longitude/latitude coordinates; and Driving Directions, which generates a route from Point A to Point B based on as much address information as you can provide. It'll get you from house to house, city to city or from a mall in Vancouver to an airport in Florida, and it'll estimate how long it'll take you to get there.

Every day, MapQuest.com generates about 5 million maps and about 7 million sets of driving directions. MapQuest deals with a lot of data -- it covers the United States, Canada, France, the United Kingdom, Germany, Italy, Austria, Belgium, Denmark, Luxembourg, the Netherlands, Sweden, Switzerland and Spain to the street level, and it covers the rest of the mapped world to the city level. Sources for this coverage include MapQuest's own cartography data developed for its print publications, information from digital mapping companies like NavTech and TeleAtlas, and government databases like the one compiled by the U.S. Postal Service. MapQuest updates its information every three months with any new or corrected data that comes in from its sources.