Cloud computing is one of the most promising distributed field, which has given the new face to the distributed field. Cloud computing has recently emerged as a new model for hosting and delivering services over the Internet. The main aim of cloud computing is to provide the resources as a services to the client. In this thesis we study about the cloud computing architecture, which includes three well known different services provided by the cloud computing, they are Infrastructure as a Service IaaS,SaaS and PaaS and such services are provided by the three different types of the cloud, they are private cloud, public cloud and hybrid cloud. Studies about the different layers of the cloud computing architecture are also discussed in this study. Two main issues. First ¿allocation of efficient virtual machine¿ and the second ¿reallocation of the Virtual Machines¿. The first issue is concern about the different load balancing algorithms, which are used for allocation of virtual machines. Somehow if the cloud fails to provide the virtual machine to the client client the concept of reallocation i.e. migration of virtual machine from one source machine to other target machine.