Looking over in Toon Zone, it seems as though there is still much confusion as to what exactly will happen on February 19. Well, in the interests of an informed public, I'm here to explain it all to you. Basically, all full-power terrestrial television stations will be required to broadcast only in digital (ATSC). Currently, most broadcast in both ATSC and analog (NTSC), while those who have gone on the air on or after 1998 can only broadcast in analog. Notice how I said "full-power" stations. That's to differentiate from "low-power" stations. Low power stations are stations that typically end with either LP or CA. The overwhelming majority of them are either shopping channels or religious affiliates. You probably don't even get any of them, because they are not covered by the FCC's "must carry" rule, and their strength can never exceed 10 kilowatts. Anyway, they are not required to convert to digital. Some will, but most will not because of the generally low-budget nature of most of them. So there's still a chance your old TV may be able to receive some kind of signal after the switch.
Which brings me to my next point. Contrary to what some believe, you CAN receive a high definition signal on a digital SDTV. Of course, it's downgraded to 480i, but you can still watch it. As long as the tuner is digital, you can receive any digital signals, regardless of format.
Now for the practical purposes. Most of the TV's out there, HD or not, will require a converter box to receive over-the-air signals. HOWEVER, if you want to receive HD signals (that is, watch over-the-air HDTV on an HD monitor) you need an HD receiver, which is entirely different and much more expensive.
Cable subscribers don't have to worry about anything, for now. All cable providers are required to provide at least the basic local channels in analog up until 2012. However, if your an analog cable subscriber without a box, be prepared to start losing most of your cable channels. The big companies will soon start moving most of the basic channels to the digital tier to free up bandwidth, so you'll need to either subscribe to digital cable, or switch to satellite.
Satellite customers are not affected at all, besides those to receiver local channels off-the-air.