Water sensors mostly are able to detect the presence of water along with checking the leakage.
Grove - Water Sensor detects the presence of water using exposed PCB traces. The sensor is made up of interlaced traces of Ground and Sensor signals. The sensor traces are weakly pulled up using 1 MΩ resistor. The resistor will pull the sensor trace value high until a drop of water shorts the sensor trace to the ground trace. This circuit will work with digital I/O pins of Arduino or you can use it with the analog pins to detect the amount of water induced contact between the grounded and sensor traces.
For all Grove users (especially beginners), we provide you guidance PDF documents. Please download and read through Preface - Getting Started and Introduction to Grove before your using of the product.