Sensing makes us aware of our ambient environment with amazing precision and speed. The influx of multitude attributes is processed in real time that makes us act in an exceptionally coordinated and timely manner. This human capability has influenced researchers worldwide trying to imitate this intricate delicacy of nature. Such a concept of global sensing and actuation is what the world has been waiting for, which has emerged in the form of Wireless Sensor Networks (WSN). This chapter delves into the whole spectrum of WSNs; explaining in detail the constituents (sensors, processor, transceiver etc) of sensor node, characteristics, survey of existing platforms, network dynamics and topology, energy matters and applications of WSNs. Applications of WSN have been given special consideration to show how WSN can revolutionize living standards, safety and our environment concurrently being more economically beneficial. Extensive due diligence has been carried out to ensure the right technology for the right application, with comparison to existing solutions and supported by real life deployments. In the end, a macro level insight of emerging branches (WMSN and WSAN) of WSNs provides a glimpse into the future of WSN.