Wireless Sensor Networks (WSNs) are an emerging technology with a wide range of potential application areas such as monitoring, tracking and controlling. For several applications of Wireless Sensor Network, security is an important requirement. However, security solutions in Wireless Sensor Network are entirely different compared with traditional networks due to resource limitation and computational constraints. This paper presents the characteristics, security requirements, encryption algorithms and operation modes of Wireless Sensor Network to develop a security solution. Also, an overview of the applications of WSNs and different attacks and their countermeasures are to be discussed.