Cloud Computing is an innovation ideas that helps in reducing the computing cost. Cloud Computing offers better computing through improved utilization and reduced administration and infrastructure costs. Cloud computing is the long-held dream of computing as a utility. Cloud Computing is the combination of Software as a Service and Utility Computing. Cloud computing shares characteristics with autonomic computing, peer to peer, grid computing, client server model, mainframe computer and utility computing. When reviewing the new cloud computing technologies, and indicate the main challenges for their development in future, among which resource management problem and job scheduling stands out and attracts the attention. Combining the current scheduling theories, the cloud scheduling hierarchy is proposed to deal with different requirements of cloud services. From the theoretical aspect, mainly accomplishes three research issue. Firstly, solve the resource allocation problem with the user-level of cloud scheduling. Secondly, address the task scheduling problem with the system-level of cloud scheduling. Thirdly, settle the evaluation problem for on-line schedulability tests.