Based off the verb "Discontinue", which means "to break the continuity of : cease to operate, administer, use, produce, or take".
For all practical purposes, it means that the item that was discontinued was produced before, but isn't being produced any longer. This often follows the item no longer being useful, like those Y2K countdown clocks that were made in 1999 - there's not much use counting down to something that already happened. Another common cause is having a more useful successor made - for example, the DC50 digital camera, which is several years old, is discontinued;, but the DC3900, a more current model in the same line, is still made. A third common cause is the item simply not being profitable for the producer - It may still be really useful and helpful to those who use it, but not enough people use it to justify its continued production.
While you can no longer get them through most stores or through the original producer, especially if the product has been discontinued for a long time, lovers of discontinued products need not fear, there are still many ways to acquire them. In the real world, many discontinued items can be acquired from things like swap meets, flea markets, and pawn shops. In the wonderful online world, Ebay (www.ebay.com) is discontinued product heaven - You can find almost anything you can think of in the tens of millions of items stored in its auction database.