Download the authoritative guide: Enterprise Data Storage 2018: Optimizing Your Storage Infrastructure
Traditionally, networks were set up as a server-client system, a server and a number of workstations or clients. All of the network storage was centralized in the server (the server did everything from file serving to authentication, email, etc.) and if the server crashed or was down for a scheduled maintenance or a storage upgrade, productivity simply stopped, as the clients didn't have any file serving or management capabilities. This is known as the server-centric model.
Obviously, server crashes or downtime due to upgrades or maintenance are undesirable for most companies. Worse still, upgrades, management, maintenance, and other routine changes must either inconvenience users or be done in the weekends or overnight, which isn't a satisfactionary solution either.
NAS is nice
With the introduction of NAS, Network Attached Storage, about a year ago, companies and individuals alike now have an easy and efficient way of simply adding network storage.
The problems with server-centric storage, combined with desire to not compromise productivity, have led to a new way of designing server-client systems where the actual storage is no longer an integral part of the server. The users' needs for immediate access to data and the ability to upgrade network storage at any given time, without affecting productivity, are much better handled by this new storage-centric model.
In a storage-centric system, the server still handles all the processing (such as needed for file serving, authentication, email, and so on) but the server doesn't have to do deliver the data to the client. Furthermore, the actual network storage can be located anywhere in the network and is not physically a part of the server, and can consist of one or multiple NAS devices.