Maximum power point tracking controllers are sophisticated charge regulators that can deliver considerably more power into your battery bank than conventional charge controllers.
Solar cells work most efficiently at a particular voltage, which changes slightly as the amount of light on the panel changes. Conventional charge controllers work at a voltage which is determined simply by the state of charge of the battery bank which is connected to the panel - but that is not always the voltage at which the panel would be giving it's maximum power output.
By employing a control circuit to search for the voltage which gives the maximum power output, and a voltage convertor, an MPPT controller can be drawing power from the solar panels at the most efficient voltage for them, but delivering it to the batteries at a different voltage. For example, in the diagram below, the Maximum Power Point for the solar panel occurs at around 17 volts, but if connected to a fairly flat battery, a conventional solar controller would pull the solar panel voltage down to around 12V. An MPPT controller could keep the panel operating at the Maximum Power Point of 17V.
The result is more efficient transfer of power to your battery - often about 25% more delivered energy! Although MPPT trackers are more expensive than conventional regulators, they have come down in price significantly in recent years, and will soon repay their purchase price on larger arrays. The benefits of MPPT regulators are greatest during cold weather, on cloudy or hazy days or when the battery is deeply discharged.
Another helpful feature of MPPT trackers is that they can use a solar panel which has a nominal rating of 24V or more to charge a 12V battery.