High definition refers to the number of pixels the camera is recording at.
You can still set the DVR to record at 720P to reduce the amount of memory it will use. However, the price of memory has significantly reduced over the years, so you may as well increase the hard drive and run the machines at 1080p 24/7.
There are 2 different types of HD cameras, either IP or HD – SDI.
HD – SDI is cabled using co-ax cable. This is the same type of cable used for analogue cameras. So if the cable is already there, it can make it much easier to upgrade the old cameras to brand new HD cameras.
Here are some of the advantages of HD – SDI
IP HD cameras are ran using cat 5 (ethernet) cable. IP cameras have my advantages, the main one being that cameras can easily be networked without having to run metres and metres of cable. This is ideal if a building has cameras on each floor, that all need to be networked to one single point.
Here are some of the advantages of IP Cameras