With the evolution of modern technology wireless sensor nodes are finding a lot of applications in day to day life starting from smart home system to military surveillance. The primary building block of a wireless sensor network is a spatially distributed set of autonomous sensor nodes or motes. In order to design a wireless sensor network it is necessary to understand the structure and working of a sensor node. The sensor nodes can be considered as tiny battery powered computers that consists of a computing subsystem, communication subsystem, sensor subsystem, power subsystem. In this paper we review the features of these subsystems so that it is easy for the application developer to quickly understand and select the type of component for building customized sensor node platform. In this paper we have studied the features of different microprocessors and transceivers properties used in sensor nodes. We also study the classifications of sensors based on applications, the relevant sensor parameters, and different storage devices with their properties. This paper can be a ready reference to beginners interested in this field. One more major problem of wireless sensor network application that should be addressed is the limited lifetime of sensor nodes due to energy constraints. We also review how energy harvesting can increase the lifetime of a wireless sensor network and the possible methods that can be implemented for energy harvesting.