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A network-attached storage (NAS) appliance can actually have both. It can be a self-contained device with built-in storage or it can be a gateway (often referred to as a NAS head) that uses a conventional external disk array connected via SCSI or Fibre Channel for data storage.
Fundamentally, any file server could technically act as a NAS device, but in reality it's different. A conventional file server runs an OS with all the extras and "fluff" that provide added functionality, allowing you to run specialized applications (databases, email, etc.). A NAS device runs on a stripped-down OS that is optimized for disk I/O performance. A NAS appliance focuses on serving storage and ensuring data protection through specific software features. System resources are therefore dedicated to I/O for better performance.
Many NAS appliances present storage as a network share. But many will also do so directly, via Fibre Channel or iSCSI, which is way beyond what any plain file server could offer. NAS appliances from companies like NetApp also use special data backup capabilities that do not necessarily require conventional backup software. Backups can be handled using Network Data Management Protocol (NDMP), point-in-time copies or data mirroring.
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