Category - What is an example of category?

Here are some things to consider: a handful of dust particles, some papers, a computer screen, some pens, a cup, and some fruit, maybe an orange. How do these things compare to one another? The fact that they are all sitting on my desk right now is the only thing they have in common. What we have here is what's known as a category, or set of things that fit together nicely because of their shared characteristics or other distinguishing features.