What is the "windows" key?

When you press the windows key, the activities overview is displayed. This key can usually be found on the bottom-left of your keyboard, next to the Alt key, and often has a window/squares icon on it. It is sometimes called the super key, logo key, or system key.

If you have an Apple keyboard, there will not be a windows key on your keyboard. The Command (Cmd) key can be used instead.