The explosive growth of data is one of the top challenges of every data center faces today. Keeping this in mind,  it becomes very important to understand the 'cloud' storage models which can help alleviate some of these problems while delivering storage with pre-defined SLAs.

This article does  a great job explaining Storage-as-a-Service,  what it is and what its not. Storage-as-a-Service gives you the benefits of having an internal service with automated, pooled storage within a private data center.

To understand Storage-as-a-Service read the blog:

http://chucksblog.emc.com/chucks_blog/2009/08/reconsidering-storage-as-a-service.html

To understand how you can implement your own Storage-as-a-Service architecture and the challenges read:

Symantec White Paper - Architecting Resilient Private Clouds