Almost everybody runs AV, or at least they think they do. On Windows, over 90 percent of all people are running AV, and the number of people who think they are is even higher.
It's far more pervasive than any other end user technology, and is far more common in people's lives than the only other security technology with fairly widespread ubiquity - the firewall.
It amazes a lot of people that AV technology is so ubiquitous, because it is so widely reviled. Technical people will often claim that AV doesn't work, and that it causes stability problems. And almost everyone will claim that it slows your machine down.
First, let's look at what AV is and how the typical technology works, then we'll look at the huge pit of suck and why those problems are there.
What is Antivirus?
You might expect me to first define the word virus as a key to understanding AV.
But AV technologies typically go beyond viruses, also trying to detect worms, botnet software, trojans, and even spyware, adware, and attack tools - even though it can be a touchy subject as to whether most things in the last three categories are bad.
For instance, McAfee (and others) have always detected the program nmap as bad, since it can be used as an attack tool, even though many, many good guys use the tool (it simply helps map out which services are visible on a network - the name comes from "network map").
The logic is basically that the average AV user shouldn't have it on his machine, and having the AV software complain about it isn't going to stop the legitimate practitioner from using the tool.
There's merit to both sides here, and in many cases things get marked bad where the decision is quite clearly a grey area.
Anyway, all these terms are irrelevant for the moment. Suffice it to say, there is a ton of bad software that you probably wouldn't want on your machine.
The industry often refers to generic malicious software as malware, and we'll use that term. Spyware and adware are sometimes in a grey area where they aren't intentionally malicious and may not be called malware, but you should get the basic idea.
AV software is software that tries to identify malware and either prevent you from installing or running it in the first place, or remove it if it's already installed.
NEXT: How does Antivirus work?