As the others have stated, TV Tuner cards allow you to watch (and yes, record) TV on your computer. There are essentially four types of tuners (here in the US) that are available:
NTSC - This is the now, defunct analog, Over-The-Air (OTA) Broadcasts of your local stations.
ATSC - This is the new digital, Over-The-Air Broadcasts of your local stations.
QAM (or ClearQAM) - This is the tuner cable companies use for all unencrypted channels they provide within your subscription*
CableCard - This is the tuner cable companies use for all content (encrypted and unencrypted) channels they provide within your subscription.
Many TV Tuner cards come with a combination of any of the unencrypted sources, so you could see a card with both NTSC and ATSC tuners or you could see a tuner card with ATSC/QAM tuners. The cablecard tuners are different. They're more set top box replacements and are not combined with any of the other tuners.
Which card you decide get will ultimately depend on your cable subscription.
ATSC TV Tuners
If you want to eliminate Cable TV completely and rely on OTA broadcasts and Internet Streams (this is often referred to as "Cutting the Cord"), you'd want a TV Tuner card that includes at least one ATSC tuner. These generally come with one or two tuners that you can use simultaneously, so you can watch one channel while recording another.
QAM TV Tuners
If you have just a basic Cable Subscription and just want the unencrypted channels (remember that these are disappearing), you'd want a TV Tuner card with a QAM tuner. Many of these cards also come with two tuners so you can still watch one channel while recording another. I would also note that you can connect the analog/coaxial output of a set top box to the input of a NTSC/QAM TV Tuner card and receive an image. However, this does defeat the purpose of a TV Tuner card and essentially turns the card into a simple Video Capture card. The set top box would have to do all the channel changing and recording set ups.
CableCard TV Tuners
Finally, if you're wanting watch and record both encrypted and unencrypted channels from your cable subscription without the use of a set top box, you'd want to look into a CableCard TV Tuner device. These tuner cards come with anywhere between two and six tuners which can be used simultaneously (assuming you have the hardware to support them). Additionally, they do require you lease the cable card from the cable company (for a small fee). These cards do come with a high upfront cost, but depending on your needs can pay for themselves, eventually. Additionally, these types of devices are not limited to just the computer they're installed into. These devices allow you to allocate tuners to other PCs or Media Extenders that are connected to your (wired) network.
Personally, I have had every type of TV Tuner card listed above. I currently have the Ceton InfiniTV4 CableCard TV Tuner card (four tuners). Now in order for me to be able to watch any cable channel (I have Comcast's Digital Preferred subscription which includes a movie channel), I would need two set top boxes. One for the living room, where the PC is, and one for the bedroom (small apartment). That extra set top box for the bedroom would cost me an additional $18/mo. With the Ceton card, I can designate one tuner to a PC (or an extender) in the bedroom and that device acts as if a TV Tuner card were physically installed. The leased cablecard from Comcast only costs me $2/month. At $16/month savings, my $300 ($200 now) Ceton card paid for itself in just 20 months. I've had it for almost four years now.
So, if you have multiple set top boxes and a wired network between each device you want to watch TV on (Google MoCA if you do not), then a CableCard TV Tuner device is probably what you're looking for.
If you need any more specific information, just let me know.
-Wolf sends
Again, the upfront costs with this type of device is huge (PCs, wired networking, the TV Tuner card itself)
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