A TiVo HD (or HDXL) can utilize a card you get from the cable company to get all of your Digital and Encrypted channels that you normally use their box for. A Series 2, can control a cable or satellite box through the built-in serial port on the box, or via IR. The TiVo HD had HD capability, and the Series 2 is SD Only.
There are several Open Source products that enable you to download and decode TiVo files for general use on your computer. TiVo itself has a deal with Roxio to allow Toast and Popcorn to download, play, and decode TiVo files. However, Toast and Popcorn also include software to play the files without decoding them. You can also store the original .tivo files on your computer, and copy them back to the TiVo for viewing on your TV.
This is all done over your home network, either wired or wireless. The one drawback to TiVo is that it only allows one download at a time, so it is somewhat time consuming, but if you have a nice queue going, your computer can be decoding files while downloading the next.
I've had TiVo since September of 2005 and I love it. I have yet to jump into the TiVo HD, as I only have one HDTV, and only limited cable, but I then when I move to my next place, and get better cable, I will do it.
TEG