AppFabric is Microsoft's brand for its [cloud computing] products. It is split into two divisions: Azure AppFabric and Windows Server AppFabric.
[Azure] AppFabric runs 'in the cloud' - it provides services that are accessed over the Internet, including a Service Bus and an Access Control service.
[Windows Server AppFabric]
Windows Server AppFabric runs 'on premise' i.e. inside an organisation's firewall as opposed to on the Internet. Windows Server AppFabric provides services for hosting and managing [Workflow Foundation] components, and also provides a [memcached]-like distributed cache.