Geocoding usually refers to taking an address or other location and turning it into a pair of coordinates. You can then plot that coordinate on an online map, or find things near it. Latitude and longitude points are really useful to programs, but not so useful for people.
Google now supports reverse geocoding. If you already have the coordinates, you can now find the address. It's not perfect, as it often gives a range of addresses, but it sure beats a long string of numbers.
They updated both their HTTP verion and the Ajax version, which means you can reverse geocode from the client side using JavaScript or the server side using whatever you want. Here's a call to the Wired office coordinates using the HTTP version. Webmonkey has a full geocoder tutorial for server-side geocoding. Note that Google returns coordinates as longitude,latitude, but expects them to be passed to the geocoder as latitude,longitude.
Google released a clickable example map to show off the Ajax reverse geocoder. Drag and zoom the map to the spot you want to reverse geocode, then click. The script grabs the coordinates and prints the address in an info window.
With the increase in location-based services, and devices like iPhones and Android phones that have access to GPS, we'll be seeing more latitude and longitude input. Already I've noticed many users on Twitter with latitude and longitude as their location, which isn't very human-readable. The coordinates are likely for computers, but perhaps now we'll have a more readable version that also works for humans.
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