This is going to be relatively lightweight in terms of technical content; most explanations of what a security exploit is do tend to be very technical in nature.
So what is an exploit? At the most fundamental, it is what an attacker uses to take control of your computer. It can be compared with the installation routine you normally run to install a new application, because an exploit is effectively how an attacker installs their agent onto your computer.
There are on a simplistic level two kinds of exploits out there – the kind that works against people (the attacker tricks the person into running their code) or the kind that works against software. Of course there are exploits that sit in the middle and defy this simplistic classification, but as this is a simplistic posting, we'll gloss over those.
Exploits against people are frankly quite boring. All that is needed to protect against them is to apply the relevant operating system patches and avoid turning off malware protection. Unfortunately people tend to be very resistant to operating system patches and often indulge in practices that turn off their malware protection (drinking!).
The technically interesting exploits are the exploits against software. And when I say "technically interesting", it means that they get very technical and difficult to describe very quickly.
But at the core, such exploits involve tricking the computer into treating what should be considered as data as code. As a very simplistic example, imagine you have a web form that takes input from random strangers on the Internet, and the input from that form is added to a database. If the code has been written naively an attacker can simply append their code to the end of the input and it will be run.
All computer data is at the lowest level nothing more than numbers. The word "Hello" is actually encoded as a stream of numbers: 72, 101, 108, 108, 111, 10. Taking just the first three numbers, and I find there is a photo of myself with that sequence. And one of the editors I use (EMACS) also contains that sequence. The meaning of a sequence of numbers is dependent on how the computer chooses to interpret it.
A great deal of computer code is dedicated to interpreting those sequences of numbers as intended – so a picture is shown as a picture and not run as code in a Python interpreter. If a computer mistakenly (or is tricked into) misinterpreting a sequence of numbers as some kind of data that it is not, then the result could be just about anything but is most likely to be a crash or displaying garbage.
And just occasionally the computer will run that misinterpreted data as code.
And that in some circumstances can be called an exploit. But what happens after the exploit? What does the attacker do then?
That's something for another time.
