Some people use the term "security system" to mean "video security system". A video security system allows you to see different views around (and possibly inside) your home. Like looking through a window, but quicker and easier. Optionally, you can record the camera images for later use.
A video security system without a recording device is useful for keeping an eye on the kids and seeing who is at the front door, as long as you go to the monitor to look.
Adding a recording device lets you "look back in time" to see what happened on any camera, at any time in the last few days.