Difference between a 2-Way and 3-way Speker?

I am planning on changing my back speakers to Boston Acoustics and I can't seem to choose between them. They are the same price but one is a 2 way and the other is a 3-way. And what confuses me the most is that the 2 way has more reviews(which probably means more people chose it) and why would there be a 2 way if the 3 way has a tweeter? Isn't the tweeter necessary?