A Video Capture Card is a TV tuner card used for receiving television signals into your computer from external devices such DVD players, VCRs, cable TV and recording them to your PC's hard drive.
There are some video cards that double as TV tuners and an example of this is the ATI All-In-Wonder series. Such a card consists of a tuner and an analog-to-digital converter along with demodulation and interface logic.
Low-end models of this capture card are missing a dedicated onboard processor and relies soley on the system's CPU.
Some tuners are referred to as combo tuners because they have two (2) separate tuners - one analog and one digital. Both tuners can operate simultaneously and this is very convenient since you can be viewing one while recording from the other.
Combo tuners are equipped with 'hardware encoders' to perform encoding thereby relieving the system's CPU.
Video capture cards are varied and have different functions. They are available in FireWire (IEEE 1394), USB and PCI formats.
Video signals from external devices such as DVD players, VCRs and Cable TV are routed to your computer via USB, Composite, S-Video or RF cables and stored on your PC's hard drive. All this processing is achieved with the help of video capture software.
To capture video from a camcorder, FireWire is the recommended port because of its fast transfer speed.
Using Capture Cards as DVR
Capturing TV is achieved through the use of the USB or PCI interface. Such an interface allows you to turn your computer into a DVR (Digital Video Recorder) or PVR (Personal Video Recorder).
You are able to time-shift by playing and pausing live TV and schedule recordings of TV shows thus emulating TiVo.
These cards are either internal or external - PCI being internal and USB external and have varying inputs and outputs. These inputs/outputs are either Composite, S-Video or RF.
After acquiring all this hardware you can now use your bundled software to capture TV or video and save them to video CDs or DVDs.