I use a computer of course, but only for fun. I'm certainly not a guru. So when people started talking about "spyware" I was a little confused. It sounded like a virus, but it clearly wasn't. No problem, I visited a site that I trust (Microsoft.com) and found a very nice lady there (in a video, of course) who told me all about it.
She said (and there was a helpful transcript window so that I could read the words as well) that spyware is software that takes control of your computer without you knowing it.
She said it can change my computer's behaviour in ways I don't want. If I got some spyware, my machine could slow down and I could begin to see pop ups I didn't want to see. Apparently, you can get spyware by downloading software that doesn't clearly state what it's going to do after it's installed.
Another sign of spyware is that it doesn't tell you in an obvious way what it's going to do to your computer. As she pointed out, if it did, then I could make an informed choice to accept it or not.