If you are given a group of things, such as triangles or people, you can classify them based on some property they have. For example you might classify people by eye color. This means you would put all the blue-eyed people in one group (class), all the brown eyed ones in a another group and so on.

You could also classify them by hair color. Then all the brown-haired people would be in a group, all the blondes in another etc.

The act of putting things into groups or classes is called 'classification'.