ZFS uses the concept of storage pools to manage physical storage. Historically, file systems were constructed on top of a single physical device. To address multiple devices and provide for data redundancy, the concept of a volume manager was introduced to provide the image of a single device so that file systems would not have to be modified to take advantage of multiple devices. This design added another layer of complexity and ultimately prevented certain file system advances, because the file system had no control over the physical placement of data on the virtualized volumes.
ZFS eliminates the volume management altogether. Instead of forcing you to create virtualized volumes, ZFS aggregates devices into a storage pool. The storage pool describes the physical characteristics of the storage (device layout, data redundancy, and so on,) and acts as an arbitrary data store from which file systems can be created. File systems are no longer constrained to individual devices, allowing them to share space with all file systems in the pool. You no longer need to predetermine the size of a file system, as file systems grow automatically within the space allocated to the storage pool. When new storage is added, all file systems within the pool can immediately use the additional space without additional work. In many ways, the storage pool works similarly to a virtual memory system. When a memory DIMM is added to a system, the operating system doesn't force you to invoke some commands to configure the memory and assign it to individual processes. All processes on the system automatically use the additional memory.