Definition:
A digital video recorder (DVR) is a device that records video in a digital format to a disk drive or other medium. The term includes stand-alone set-top boxes and software for personal computers which enables video capture and playback to and from disk. Some consumer electronic manufacturers have started to offer televisions with DVR hardware and software built in to the television itself. It has also become the main way for CCTV companies to record their surveillance, as it provides far longer recording times than the previously used VCRs.

Functions and Types:
There are many types of DVRs, however functionality and specs requires attention from potential buyer. CCTV DVRs divided into 2 major categories stand alone and PC based.
Standalone DVR is enclosed unit and require minimal knowledge of computer, any user would be able to operate it after quick training. However since most of the functions preprogrammed and hardware is not exchangeabe it has its limtations.
PC based DVR will require more advanced knowledge, however it will allow user to modify hardware for expansion or have multiple hard drives (HDD), etc...
in term of cost standalone DVRs tend to be cheaper for 4-6 camera setup, but price equates when number of suported cameras increase from 8 to 32.

Standalone DVRs can be small for mobile operations used in the car for wireless surveillance or relatively big for in-house operations. currently most of them support HDDs up to 400Gb and higher. more expensive models come with build in CD/DVD writers for quick back-up as well as USB port.

Depending on how many operation modes DVR support they can be classified as singleplex or multiplex. Singleplex will suport only one operation at the time, either record or review or display. Multiplex will allow all three functins at the same time.

Majority of the DVRs allow user to play with quality of the image (# of pixels) and quality of streaming refered to as fps (frame per second - 30 FPS considered movie quality).

Modern DVRs have folowing functions:
* Remote surveillance - user can watch image remotel through internet;
* Programable recording - user can specify at which time intervals to record;
* Motion triger recording - user can specify if recording required ony when motion detected, this allow to record only when activity observed, which translates to saving of HD space and increase of recording time;
* alarm triger recording - user can specify if recording required only when alarm switch trigered;
* Covert recording - user can record camera, but not show it ont he screen or cover it with blank image;
* Audio in and out support - for recording Audio from at least one camera;
* VGA support - allows connection to computer monitor.