Cloud computing is a recently developed new technology that provides a model for resource sharing over the internet, which is different from the resource sharing of the grid computing systems. Cloud computing is designed to deliver computing resources as a service to consumers over the internet from large-scale data centers – or “clouds". Reliability is one of the major issue in the cloud computing. Cloud reliability analysis and modelling are not easy tasks because of the complexity and large scale of the system. This research paper is the systematic review on the cloud computing basic concepts. This research paper also analyzes the key research challenges present on reliability of cloud service.