The reply above was 100% correct, but I thought I would add a couple of things that I would have wanted to know, prior to making any purchases...
1.) If a TV is a true HDTV, it comes with a built in HDTV tuner. BUT, that tuner is only good for over the air broadcasts (usually). It usually will not work with any Cable TV's HDTV signals (yet), and it will definitely not work with DirecTV or with Dish Network.
2.) If a TV says it is HDTV-Ready, or HDTV-compatible, that means it has no HDTV tuner at all; that is the type of set you mentioned, of course.
3.) Here is where it gets a bit tricky.... if you decide to buy an HDTV-ready set, AND you decide to buy an HDTV tuner as a separate set top box...
-a.) There is one type of HDTV box that will only let you receive HDTV over the air signals. It is identical (essentially) to the ones built into number one, above.
-b.) There is a second type of HDTV box which will let you watch DirecTV HDTV signals, AND it also does double duty and lets you receive over the air signals. If this is the final plan, you may not want to waste your money on a true HDTV set, and you may not want to waste your money by buying the type of HDTV box mentioned in "a.)" above, even as a temporary solution, unless you will want to E-Bay that first box later. Your new DirecTV box will do everything the box in "a.)" does.
-c.) There is a third type of HDTV box which will let you watch Dish Network HDTV signals, AND it also does double duty and lets you receive over the air signals. If this is the final plan, you may not want to waste your money on a true HDTV set, and you may not want to waste your money by buying the type of HDTV box mentioned in "a.)" above, even as a temporary solution, unless you will want to E-Bay that first box later. Your new Dish Network box will do everything the box in "a.)" does.
-d.) There is a fourth type of HDTV box which will let you watch Cable TV HDTV signals, but the Cable companies usually rent these to you, or include them in the overall cost of their HDTV package. Some of these Cable TV HDTV boxes also do double duty and will let you watch over-the-air HDTV, but many of them will not. So, in this case, planning is really tough.
Just some thoughts to ponder...
-Bruce in Chi-Town