A clustered NAS system is a distributed file system that runs concurrently on multiple NAS nodes. Clustering provides access to all files from any of the clustered nodes regardless of the physical location of the file. The number and location of the nodes are transparent to the users and applications accessing them. Clustered NAS systems are distinguished by their large file systems that can scale to hundreds of terabytes (or more) of addressable capacity.
Clustered NAS systems typically provide transparent replication and fault tolerance, so that if one or more nodes fail, the system continues functioning without any data loss. Data and metadata can be striped across both the cluster and underpinning block (DAS or SAN) storage subsystems. Although clustering appears similar to file virtualization, the key difference is that with a few exceptions, the system nodes must be from the same vendor and be configured similarly.
If you're considering NAS, this guide on network-attached storage devices walks you through the decision process.