Windows Azure AppFabric is best described as a complete cloud middleware platform to develop, deploy, and manage applications on the Windows Azure Platform. It also offers you many benefits like adds higher level PaaS capabilities, on top of Windows Azure application model, bridges existing applications to cloud and provide consistent development model for Windows Azure as well as Windows Server, through secure connectivity across network and geographic boundaries.
If you wish to get certified in Azure, enroll in a good Azure training from a reputed e-learning institute like Intellipaat. As they offer one of the best industry-grade courses with a host of benefits like Lifetime 24*7 technical support and access, Lifetime free upgrade, 1:1 doubt clearing sessions, etc. Also, look at the following video on Microsoft Azure Certification Preparation and Real-World Roles to get a better idea.