Analog sensors are devices that are capable of returning an analog signal that is proportional to the measured entity. Then, the analog signal should be converted into a digital one using proper circuitry or, for simplicity, an AD.
A moisture sensor is a device that can change its internal resistance according to the wet intensity of the soil moisture:
The device can be purchased at http://www.cosino.io/product/moisture-sensor or by surfing the Internet. The user guide of this device is available at http://seeedstudio.com/wiki/Grove_-_Moisture_Sensor .
This device is quite similar to the water sensor presented ...