Left un-compressed, raw footage coming into a DVR from the cameras would soon fill up the hard drive. To get round the problem programs called codecs have been developed to compress the data so it takes up less space. The most common codec used in DVRs is H.264
Compression codecs are very common. JPEG is a well known compression method used in still images. H.264 was developed by 2 organisations and is also known as MPEG-4 AVC. H.264 is most commonly used in Blu-ray discs and streaming video such as Vimeo, YouTube and iTunes.