The difference in the resolution that your cameras offer versus the resolution that your DVR displays and records are often confusing for the end user. It doesn't do any good if you have the highest resolution camera available if the DVR that it's connected to can't take advantage of the increased video quality.
A camera's resolution is the measure of the detail that it will capture. This is typically measured in Television Lines(TVL). With a higher TVL rating, you'll have better definition, clarity and picture quality. The camera scans an image in a series of lines running horizontally. Each horizontal line is made up of a number of elements. Resolution is a measure of the quantity of both the horizontal lines and the elements in each line. The TVL has a direct correlation to the number of pixels on the CCD image sensor. Resolution measures the number of horizontal lines a camera uses to produce an image. Horizontal resolution measures the number of elements making up each horizontal line. Vertical and horizontal resolutions usually yield a 3:4 ratio relationship (for example, 600 vertical lines to 800 elements in each line). The higher the camera's resolution, more detail is visible because the lines are closer together and there are more elements in each individual line.
DVRs today offer several different resolutions, these range from CIF (360?240) on the low end to D1 (720?480) which is the highest available in the CCTV industry today. This measurement refers to the size of each image or frame that the DVR displays and records. Sometimes a DVR will offer a higher display resolution then what it can record to a storage device (hard drive). This is definitely something that you want to be aware of, you'll want your recorded images to be of the highest possible quality because your stored video footage will be what you'll give to the police if the need arises. How many frames per second (FPS) your DVR can record at is also very important, as this will affect the smoothness of the footage. 30 fps is the industry standard for real time video, meaning that it will offer video that is as smooth as if you were viewing it with the naked eye.
If you have higher resolution cameras (520 TVL- 700 TVL), you'll want a DVR that can record the frames at a size that will allow you to take advantage of the increased detail. A system that can record D1(720?480) size frames at the 30 FPS is great way to achieve this goal.