Analog video has been the standard in the security system industry for many years. It uses coax cable, which many homes and businesses already have. However, analog video doesn't have the definition that has become the industry standard. If you have cameras that are more than 5 years old, chances are they're out of date and can be easily upgraded to analog HD.
With analog HD video, you can blow up a screenshot to around 3 megapixels, compared with the newer technology of IP video, where you can blow up to 5 megapixels. However, if you're looking to upgrade your current camera system, analog HD is the way to go. You can use your current coax cabling and tower wire, so there are lower start-up costs compared to replacing your camera system.
HD video is a bridge between the older analog video and the newer IP video. It uses your current infrastructure so it costs less than getting a new system. And, you'll get better resolution with HD than with analog cameras.