Wireless sensor networks (WSNs) are nothing but collections of various inter connected sensor nodes which cooperatively send collected data towards the sink or base station. Efficient and prolonged use of sensor networks require low traffic within the network, no congestion causing scenarios, minimal use of energy, avoid redundant data transmission and minimum number of data transmission towards the sink. To achieve all these things effectively data aggregation plays a very important role. The main aim of data aggregation is to gather and aggregate data in an energy efficient manner along with removing redundant data to improve the lifespan of the network. Hence in this paper, the review took on different data aggregation approaches such as flat network, hierarchical network and structure-free network and their several subtypes. This structure based and structure-free data aggregation approach uses lots of different types of energy efficient aggregation protocols for performing aggregation on data. Hence, this review paper mainly concentrates on various data aggregation approaches along with their different data aggregation protocols.