A 3d, or three dimensional, printer is a printer that uses plastic instead of ink. While a "normal" printer places droplets of ink on a 2-dimensional piece of paper, a 3d printer deposits drops of plastic. The 3d printer can add more drops to a given location, building up the layer of plastic into the 3rd dimension. Through this process, the 3d printer can be used to create any object that can be made from the print material.
Initially, the cost of these printers was in the hundreds of thousands of dollars, but has now fallen into the thousands of dollars, and may soon fall to the hundreds of dollars... comparable in cost with home printers. As printers enter the consumer market, they are curiosities and can do amusing things, such as build plastic cups and statues. As time goes on, however, they will become capable enough to build virtually anything found in a home.
Just as home printers evolved from black and white to color, and eventually led to color printing and later photographic printing, so too is the 3d printer quickly evolving. Today the "ink" is usually a grey plastic. However, colored plastic cartridges are available for higher-end models, allowing for a greater range of products to be produced. Electrically conductive "ink" is being developed that will allow printed parts to actually work. For example, you could "print" a radio, or a toaster oven. Still other experiments are being conducted to create flavored, edible "ink," allowing you to "print" food.
As 3d printers evolve in capabilities, and as "ink" takes on new and wider functions, almost any conceivable product can be printed on demand. With this capability, the 3d printer becomes very reminiscent of the fictional Star Trek replicator, able to create anything you need out of thin air. This completely changes the idea... and the need... of a factory. Outsourcing becomes one of the industries that could be wiped out by a world full of 3d printers. For this reason, the 3d printer may be the most disruptive 21st century technology to date.