CableCARDs are computer cards that can be put into certain televisions and other related devices (think TiVo series 3). They essentially replace the set-top cable box and allow the television to directly manipulate the cable signal. The CableCARD is provided by the cable TV company and has a standardized interface so that any device with a CableCARD slot can accept any CableCARD.
The reality is a bit different. First, CableCARD acceptance is low: 200,000 users out of the millions of cable TV subscribers. This is due, in part, because only the newest television devices are compatible with them. Thus you have to purchase a new TV or TiVo to use the CableCARD. In addition, the current crop of CableCARDs doesn’t support two-way communication (think Pay-Per-View, interactive programming guides, and video on demand), so some viewers are sticking with the set-top box even though they could use the CableCard. Second, there are incompatibilities between various CableCARDs and the various television devices, meaning you may not be able to use a CableCARD in your brand-new large screen TV.