What's the difference between a woofer and a tweeter?

It is commonly thought that there will always be a phase difference of 180° between the outputs of a (second order) low-pass filter and high-pass filter having the same crossover frequency and hence in a 2-way system the high-pass is usually inverted to correct for phase. For passive systems the tweeter is wired opposite to the woofer and for active systems the high-pass filter's output is inverted. In 3-way systems the mid-range driver or filter is inverted. This is generally only true when the speakers have a wide response overlap and the acoustic centers are aligned.