Sensor is a material or passive device which changes its conductive properties according to a physical stimulus. sensor node is a small device that features sensors, a processing unit and a communication module. Sensor networks allow for automatic delivery of the infromation from our environment. Sensor deployments are performed by a number of European and US institutions. 112 types of sensor nodes, 118 types of processing units and 112 types of communication interfaces are identified in the research community. Sensor middleware supports the operation of the sensing-based applications. Sensor data includes measurements performed by sensors (pressure, luminance, noise, CO, CO2...). Sensor metadata is data about the sensor nodes or the sensor network.