The purpose of this white paper is to provide a semi technical explanation of what a capture card is and some common uses for such.
(links from this page are to Wikipedia)

Other common names for a Capture Card are:

       Frame Grabber
       Video Capture Card
       Video Acquisition Device
       Video input device
       Capture Board

Scope of this article:

There are “commercial grade” and “consumer grade” capture cards, this paper will be commercial grade focused but general principals will apply to both. References will be to commercial grade devices and software.

A capture card is an electronic device that accepts a single or multiple video or video and audio signal(s), digitizes them and makes the captured signals available to other hardware or software for further processing such as  display on a monitor connected to host system, compressing, streaming, transmitting, recording or some combination of above.
A capture card is typically a PCI or PCI Express type printed circuit board that resides in a host microcomputer systems slot. So a capture card is a component of a larger system that performs some specific function. (Note: There are standalone capture devices that typically interface with microcomputers via some data interface connection such as USB. This paper will not explore standalone devices, rather the card or board types.) Depending on the manufacturer of the capture card, multiple cards can reside in a single host system allowing multiple concurrent video/audio streams to be captured and processed. In addition various capture cards can accept different analog or digital signal types. e.g. HDMI with digital embedded audio, DVI digital or analog, VGA analog, composite video, component video, serial digital video (SDI), DisplayPort etc.

Picture of a Capture Card:

                                                                                                 A Datapath VisionRGB -E2s Capture Card
What is a Capture Card and why would anyone want one?

A white paper on the subject of video and audio acquisition
(Draft Document in progress)
December 4, 2014
Video Wall Controllers:
Video walls are generally driven by a controller or processor, where many different real time video signals are captured and aggregated inside of a special built computer system called a controller. Then the viewers of the video wall can determine via the controllers software which signals being captured will be displayed on the video wall and in what size or scale they are presented in. Various capture window layouts can be saved and recalled for convenience in different viewer scenarios.

                                                                                                A Pixell VP-2000B video wall processor:
Rear of processor showing                                                         Front view
Various signal type capture cards
Telemedicine for remote medical and surgery:
Long distance administering of medical services for patients in remote locations.
Purpose built systems that have cameras and medical monitoring devices need to have their outputs captured for display to the surgeon to view and to be recorded for long distance and forensic purposes.

                                                                                                 An Intuitive Surgical® da Vinci robotic surgery system:
Camera aided manufacturing and inspection:
Dedicated systems that can measure and inspect precision parts being manufactured with high resolution cameras. Systems typically have recording features for forensic purposes from the signals captured with capture cards.

                                                        A Boulder Imaging ML-12 measuring system:                     A Manncorp™ Sherlock-300B PCB inspection
Entertainment and live shows.
Live event production companies use capture cards to “cut in” pre-recorded video clips to be superimposed and displayed during live performances, and then the whole performance is captured and recorded.

                                                                                                               A live concert with light show:
Common video signal types captured:
Component YPrPb (RGB analog)
Composite SD video
HDMI Digital HD video (with embedded digital audio)
DVI Digital video
DisplayPort HD digital video
Analog Audio
Serial Digital Interface (SDI)

Because capture cards reside in a host computer running an operating system (typically Microsoft® Windows® or Linux) the card manufacturer must write drivers specific to their hardware so the host OS knows about the hardware and allows it access to system memory and bus allocation resources. The integrity and stability of these drives is paramount as capture cards are used many mission critical applications and any bugs or memory leaks will cause a complete system failure and quite likely will cause an OS system crash.

Interfacing capture card to system builder software:
Since there is typically system builder software that will need to take the captured video stream and do further processing on it as above, most capture card manufacturers offer a simple to write to interface standard such as DirectShow from Microsoft. If the manufacturer offers a software development kit (SDK),
this is typically a more complex and difficult to write but faster method to acquire the video stream.

This paper is working draft in development, written by James Thornburg of Pixell.
Since capture cards are not finished products in and of themselves, but are components used in larger systems, it might be useful for the reader to learn some examples of what these systems might be used for:

Common System Functions That Capture Cards Would Be Integrated Into:

Lecture capture systems:
Computer systems made for electronic remote presentation and preservation of live meetings.
Its function would be to capture, record, stream live over a network or the internet (for remote attendees) live lectures or meetings in a university, or corporate environment. A common scenario would be that the presenter would have a camera on them, their laptop with their presentation on it, and the audio from the presenter’s microphone would all need to be captured in real time by the capture card(s) in the lecture capture system. The manufacture of the systems software would then allow meeting coordinators to simply save the lecture to disk for future viewing, while compressing and streaming the lecture live. (Another similar application would be for the production of MOOC’s.

                                                                                             A MediaPOINTE® DMR HD PRO lecture capture device: