Spyware is an all-too-common cyber threat encountered by internet users. According to Malwarebytes, four in five internet users currently have spyware on their devices. Spyware, of course, consists of software that’s designed to log, track or otherwise spy on a user’s activity. It’s been around for decades, with some of the first known spyware originating in 1995. Since then, it has become more common. While there are hundreds if not thousands of different types of spyware, though, most types fall under one of four categories.

#1) Adware

Adware is a category of spyware that involves the use of ads. Like all spyware, it still spies on a user’s activity. But adware uses this data to serve ads. It may track the websites visited by the user, after which the adware will serve custom ads based on these websites. Of course, not all adware is spyware. Only adware that logs, tracks or otherwise spies on a user’s activity is spyware.

#2) Web Tracking

Web tracking spyware is designed specifically to track users’ online activity. It typically involves the use of cookies. When you visit a website, the site may deploy a cookie. Your web browser will download the cookie — a small text file — and the website will use it to track your online activity. As long as the cookie remains present, the website will see your online activity. This is a form of web tracking spyware. Web tracking spyware allows websites to track users’ online activity.

#3) Trojans

Trojans are commonly used as spyware. Trojans are malware items that are designed to sneak into a system undetected. They are typically disguised as safe, legitimate programs. You may download and install a program, only for it to infect your computer with a trojan. Trojan spyware is no exception. The only difference between traditional trojans and trojan spyware is that the latter is designed to spy on users’ activity. If your computer is infected with trojan spyware, it may discreetly spy on your activity.

#4) System Monitoring

There’s system monitoring spyware as well. System monitoring spyware is designed to monitor an entire computer system. It’s arguably one of the most troubling categories of spyware. While other categories of spyware may only capture specific types of information about your activity, system monitoring spyware will capture everything you do on your computer. It can see what programs you used, what websites you’ve visited, what keys you’ve typed and more.