The moisture sensor board features both analogue and digital outputs. The analogue output gives a variable voltage reading that allows you to estimate the moisture content of the soil (using some math!). The digital output gives you a simple "on" or "off" when the soil moisture content is above a certain value. The value can be set or calibrated using the adjustable on board potentiometer.
The sensor features a nickel plated anti-rust spade, and a wide sensor to ensure accurate moisture readings. The module uses the LM393 "comparator" chip-set to measure the moisture content.