The "tower" is the component of the computer (desktop computer) that houses all the actual computer hardware. It stands separately from the monitor and is what you normally would plug everything into. For example, this is a tower: [link] I hope that helps.
In my grade 8 class, NOBODY actually knows the qwerty keyboard setup (seriously? When did they stop teaching that in school???) so I rearranged all of the keys on every keyboard, and nobody could figure out how to type. It took hours to get all the keys on the right places again, because they had to test out every one to see what letter it really was.
The monitor is the screen, but the tower is the actual computer part. It stands separate (thus the name tower) or the monitor and that's where everything usually plugs into. Unless you just have a laptop, then everything's built in together.