As a probable replacement for the cable box, the CableCARD--the term coined by its designer--is similar to a PC card you insert into a laptop, and will reduce the number of boxes you need to keep connected to your TV.
Keep reading below for an uber-quick overview of the technology, and check out this article from Electronic House for everything you need to know about this up-and-coming technology, including its draw-backs.
With the card activated and installed into your TV, you can receive analog, digital, high-definition and premium programming (HBO, Showtime, etc.). Due to its technology, it also doesn't leave the door open for any degradation of the signal as might be the case with an external solution. Limitations currently include only one-way communication, so you can't order movies-on-demand or have multiple video streams at the same time.
Connectors at one end of the CableCARD mate with connectors in a receiving device, which is often a TV, but can also be a digital video recorder (e.g. TiVo Series 3), a media center or a set-top box. The card has multidirectional communication ability but manufacturers have not embraced this and currently have limited it to uni-directional communication.
How prevalent is the technology? There have been 229,000 cards "deployed" and there are 26 manufacturers that have certified or verified about 550 Unidirectional Digital Cable Products (UDCP), such as digital-cable-ready DTV sets, for use with the CableCARD.