exactly. A PWM controller is the cheap, common charge controller that basically closes a contact when the solar panels are producing enough power to start charging the
batteries. If the voltage on the panels drops below the charge setpoint in the controller, the controller will stop charging. The solar panels are force to operate around the same voltage as the batteries. thus current in = current out.
A MPPT, Maximum Power Point Tracking controller, calculates the voltage at which the panels will produce the most power. It then uses the electronic controls to hold the voltage at that point. The controller is able to use the extra power to put a higher charge into the batteries or for house use. Since the max power point constantly changes with solar intensity and panel temperature, the MPPT controller constantly "samples" the panels. Its a pretty efficient system. A way to explain it is since the solar panel can only put out a certain amount of power (P) and the CC controls the panel output voltage (E) the controller can essentially control the current (I). the formula: P=I x E. In other words: I(charger output current) = P(panel power at a given time) / E (the voltage that the CC calculates is the max power point at that time)
That's a simplified explanation. I'm no expert. I don't know exactly how the CC does it.
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.