Barb,
Depending on what you're looking for. If you justb want to monitor who's at the front door, a simple wired camera will plug into your TV. If you want more cameras, there are a couple 'plug-ins' on the TV input like A/V 1, AV 2, component 1, component 2, all using RCA plugs. Then there are HDMI and S video using different plugs.
If you get in to it, there are routers with 500 GB and 1 or 2 terabyte hard drives.
I've had security cameras for over 20 years, all using the cheapie cameras. All work perfectly well in the day time. Night vision is limited by the number and quality of infra-red lights and the number of 'TV lines', though most of my cameras are 500 lines and 700 lines were disappointing.
I have two 8 camera router/recorders around the house. One just for the chickens. Got a few new chicks that are fun just to watch their antics.
I have 5 on my tow van which doesn't have any windows behind the cab so there's 3 monitors in the cab.