SD CCTV was invented back in the 40's, and up until a few years ago it was the only choice when looking for a surveillance system. Due to the fact it has been around so long, it is fair to say it is an extremely reliable format, it also has by far the widest variety of products - such as cameras shaped like smoke detectors or PIR alarm sensors.
SD CCTV transmits an image from a camera to a monitor / recording unit. Traditionally this was always done using RG59+power Coax cable,RG6+power cable - however in recent years it has been possible to use Cat5 cable as long as you use Cat5 baluns.
The most important feature to be aware of is "Standard Definition" - SD CCTV is limited in the image resolution it can produce, it provides image resolutions much lower than HD. Most good SD CCTV systems will produce D1 resolution images, the very latest and highest possible image quality in SD CCTV is called 960H.
D1 Resolution - is 720 x 576 pixels - that's a total of 414,720 pixels
960H Resolution - is 960 x 576 pixels - that's a total of 552,960 pixels