An antivirus is a program that can help you protect your computer against all types of viruses, malware, trojans and other kinds of undesirable visitors that are dedicated to infecting your computer.
A virus can act in various ways, damaging and affecting your computer or phone with reduced performance, data alteration, file deletion, even uninstallation of the operating system, just to name a few. Over the years, these viruses have become more sophisticated, therefore, your antivirus must be updated at all times and should be just as sophisticated. The purpose of an antivirus software is to monitor everything going on in your computer, especially what a possible virus is doing in real-time. This is done with periodic checks, that will see what changed between them in order to detect malicious action and see if you're protected or not.
One very important thing to remember is that viruses are absolute, either you are protected or you're not, anti-virus either protect you or you lose everything. This is important for those who think a free anti-virus will protect them, "enough". There's no such thing. Specially with free anti-virus, most likely if it caught one virus there are 5 others it didn't.
There are both free and paid antivirus programs, but what are the differences between them?
Here are some of the main differences you may encounter, although you can expect them to vary between different software providers.
Support: it's the main difference, it is also the number of services offered by each program.
Consumption of resources: paid antivirus programs usually have a very simple window, so they will require fewer resources from the laptop. So they give you the best protection with the lowest data consumption.
Updates: the paid antivirus updates automatically without interrupting what you are doing, something that does not happen in the free version. The updates are usually more frequent as well and more in depth, covering all the latest threats and virus still mostly unknown to the general public.
Ads: in the paid version you won't have ads, something that does not always happen in the free version.
Technology: they have specialized systems for all kinds of viruses that may appear, this if we talk about paid antivirus, but free ones usually only work against most known viruses.
Support programs: free antivirus usually doesn't give inform you when something you're installing wants to install any type of program that complements itself, spometimes without your permission or with a cleverly hidden check box. Has it ver happened to you, for example, installing something and suddenly bing is your search engine? or something weirder? That's what we are talking about.
Protection of financial operations: this service is only available for paid antivirus, the free ones rarely if ever offer this function.
Parental control: the free versions do not provide you with these types of services, usually only the paid versions.
WIFI security: most of the times this service does not come integrated into free antivirus and if it does, it's in a very general way. In paid versions, this can be an extra service that gives you a best bang for your buck.
Now, at this point you might be convinced that your free anti-virus isn't enough, so what to do next? You can check this article: "How to choose the perfect anti-virus for me".
And remember, protection costs less than your daily cup of coffee and is a better long term investment.