There are a few types of TV tuner cards that I know of. I'm in the U.S., so things may vary slightly by region, if you're not. Here we have the older style cards meant for receiving analog NTSC broadcasts. I would say those are mostly phased out, due to the government's switch to digital ATSC broadcasts. Since then, cards have been designed to receive over-the-air digital broadcasts. These cards however are not designed to receive all cable television, just the channels being sent using clear-QAM, or essentially, not part of a higher tiered cable package. Nor are they designed to receive satellite service for pretty much the same reason. For proper reception of paid cable channels, you want to choose a tuner that has a cable-card slot built into it (may not be applicable outside the U.S.) This allows the card to access your particular cable subscription package and gives you the full range of channels which you pay your cable provider for.
As for HD broadcasts, digital broadcasts can be either HD or SD, and it's really not up to the TV card to determine what is what, or how it's played back. The ability for playback of HD broadcasts will be dependent on your video graphics hardware inside of your computer. The TV card is going to capture the over-the-air stream or cable stream, and dump the video data most likely in an MPEG format, either to a recorded file, or buffer it in a temporary file so you can just watch it live. The decoding of the video should actually be done on your graphics card by it's video decoding engine. If you have an older machine, with no HD capability, it is possible to be able to watch SD content and have poor playback of HD content. Of course, that doesn't mean you can't record the HD content, provided your storage subsystem is fast enough, just that it won't play back on the older graphics equipment.
There are external cards for TV, but they can be problematic. I have seen 2 connection types, USB and Ethernet. Both I have read plenty of problems concerning, and even experienced a few myself. USB tends to be low powered, and as such, you can end up starving cheap tuner equipment, having frequent dropouts of the device itself or simply poor reception. Ethernet devices I have only read about, but it sounds as though the connection to Ethernet based tuners can be lost from time to time. I'm not always the happiest person when my favorite TV show didn't record, so I always opt for internal tuner cards. Of those variety, you can have PCI based or PCIe based. Either works fine. PCI is more than fast enough for that sort of application.
Something to think about is how many simultaneous streams you want to be capturing or working with at one time. You can find tuner devices with 1, 2, or even 4 tuners built into them. This essentially lets you record up to 4 independent channels at once, while watching any 1 of the four channels as well.
If you are running Windows Vista or Windows 7, you should have Windows Media Center already built-in. Microsoft has done a bang-up job with their Media Center support for TV tuner cards in Windows. It comes with a built in program guide which lists shows for the next 1 - 2 weeks, has scheduling conflict resolution when recording programs, has the ability to wake your computer from sleep to record your programs and then put the computer back to sleep afterward, and is so seamlessly integrated that it can record TV programs without you even being aware it's happening.
You need to be aware however that while Media Center is available for Windows 8, it does not ship with it, and is only available as an add-on for Windows 8 Pro, unless something changed since I last checked to make it available for the standard edition of Windows 8.
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