There are many ways to squeeze as much footage as possible onto a given size hard drive space. This is called compression. Without compression the hard drive space would fill up exceptionally quickly.
The same is done with photographs – the most common format is JPEG which is actually a compressed version of the true image allowing it to become a much smaller file size.
H.264 is the industry standard for security camera systems and the way they squeeze as much footage as possible on the hard drive and is what you should look for in a recorder.
H.264 is also known as either MPEG-4 AVC or MPEG-4 Part 10.
There are more storage-size efficient methods of squeezing footage onto any given hard drive size. These require much greater processing power and generally aren’t currently practical within a security camera recorder.