Cloud-based storage services broadly break down into three categories, defined by use cases and software requirements: raw storage, backup and business continuity, and file synchronization and collaboration. But there's plenty of feature overlap--many backup products offer mechanisms for ad hoc file sharing and mirroring, and raw storage services can certainly be used to roll your own backup process.
Storage-centric application services higher up the value chain are optimized for a specific set of needs and usage scenarios. Raw storage services, of which Amazon Web Services' S3 (file/object store) and EBS (block storage) are the quintessential examples, essentially amount to having a hard disk in the cloud, where the service acts as an alternative to a networked block (SAN) or file (NAS) device. They essentially serve as storage containers that other applications use; many backup and synchronization services, including Dropbox, use S3 as their storage repository.
Although raw cloud storage is often used with cloud-based applications, it can also serve as an off-site storage repository for a DIY enterprise backup and archiving system--as an alternative to a remote tape vault, for example. Yet as David Chapa, Quantum's chief technology evangelist, cautions, application developers must carefully consider how they incorporate cloud storage since access methods and performance profiles will be quite different from LAN- or SAN-based systems.
Moving up the application stack, you'll find backup, archive, and business continuity services. These are nothing more than software-as-a-service apps in which cloud storage is one component of a larger offering. Like all SaaS products, the cloud hosts the storage as well as the application code, software configurations, and operations staff.
Services are further differentiated between those designed for backup versus those mirroring live data.
Backup services may be optimized for client PCs and, increasingly, mobile devices, servers, and enterprise applications. Here the choice is driven not only by the type of client device but also by the type of data you're dealing with--office files and email versus VM images and databases--and its importance and sensitivity. IT teams also must consider the standard backup requirements of RTO and RPO (recovery time and recovery point objectives) for each data type.
File synchronization and collaboration services mirroring live data were initially aimed at consumers, but, like many online utilities, they've become indispensable business tools. They're most frequently used to share office documents with business partners and synchronize to-do lists among PCs, tablets, and smartphones. Other creative applications include synchronizing system and application configurations, such as custom spelling dictionaries. As synchronization services mature and the cost of storage decreases, many of your end users will find them a suitable repository for their personal files--ensuring their information is always available no matter what device they happen to be using.