A home security system is a group of devices that all work together to protect your home, usually using a combination of visual surveillance, motion detection, audible alarms, and system alerts. The average home security system typically includes a camera or two, some motion sensors that can make out infrared energy, a handful of sirens (either built into the sensors and cameras or standalone devices), and a base hub syncing all of the hardware. You'll control the latter manually or using a companion smartphone app.
From there, most home security companies will give you the option of fleshing out your system with à la carte accessories for further protection. Popular add-ons include:
A glass break sensor that alerts the system when it detects the sound frequencies or vibrations of shattered glass.
Environmental sensors that can detect water leaks, smoke, and carbon monoxide.
A keypad or key fob you can use to manually arm and disarm the system, in case you don't have your smartphone on hand.
A panic button that quickly alerts law enforcement in emergency situations.
Yard signs and stickers that serve as visual (and inexpensive) deterrents to wannabe intruders.
Most home security companies are also in the business of home automation these days, which means you'll be able to control your setup using smart assistants and connect it to other devices like smart locks, lightbulbs, and thermostats. (Amazon Alexa and Google Assistant compatibility are pretty standard nowadays, while Apple HomeKit options are fewer and far between.) If you want to get really fancy, several brands also offer support for "If This Then That," or IFTTT, a free web-based service that lets you connect and build commands for internet-enabled devices and apps.