In the current era of technology, the number of users connecting to the internet has increased rapidly. If the situation is like Pandemic (COVID-19), the demand of the data services is much higher than the expectation. The major problem of this situation is load management. SDN plays an important role in this regard. Being a programmable network, it is widely admirable for its network intelligence. In this paper, we discuss SDN architecture, its components, various applications and major challenges. The unique architecture differentiates SDN from traditional network. It makes it possible to take routing decision without depending upon the underlying architecture. We also present a systematic review on the study of the different load balancing approaches, existing solutions and various load balancing techniques in SDN. With the network intelligence, SDN is able to take the load balancing decision. We categorize the load balancing techniques based on controller, server, communication path selection and cloud-based environment. Finally, we present different research challenges, existing solutions under each technique.