First a few definitions. Cloud storage is storage that is accessible via the Internet usually from a device like a laptop but it could even be from an iPhone. As with most "cloud" related technologies, the location of the storage is not usually an issue.
Cloud computing is essentially a more advanced form of web hosting although the computation can be more than just delivering web pages to clients. The Amazon Elastic Compute Cloud (Amazon EC2) and Eucalyptus are examples of cloud computing infrastructures that are actually compatible. The Eucalyptus Public Cloud (EPC) is an open source version of Eucalyptus.
Essentially these platforms split the computation side from the storage side providing elastic compute (EC2), elastic block storage (EBS) and storage (S3) linking them all via the network. The key to the network support is that it is essentially a virtual private network (VPN) that can be linked to the Internet or other networks. The components are usually managed as a group and can be scaled as necessary with commercial ventures charging based on various usage metrics.
But back to cloud storage. Cloud storage differs from EBS and S3 in that the storage is normally access from a device like a PC or smartphone but, in this case, the storage is viewed as logical shares as if they were on a network file server. The client devices need a mechanism to access the data and manage the storage but it is at a higher level than something like EBS.
Internet-based backup services have been common for a number of years and they fall into this cloud storage category. Normally the backup is made from a PC and stored on an Internet-based file server. Various applications are used to provide the backup and restore facilities. Some are browser-based.
The up and coming cloud storage is often called Plug Computing, a term coined by Marvell Technology. Some Marvell's Armada line of microcontrollers and microprocessors target this environment and its SheevaPlug was the basis for a number of Plug Computing platforms.
Products like CloudEngines PogoPlug, Seagate DockStar, Ctera Networks CloudPlug and Tonido TonidoPlug fall into the plug computing category. They are all based on an Armada chip and provide similar although not identical feature set. They also tie their hardware to Internet-based services that allow users to access their data via the Internet.
The basic system is essentially an Armada chip that incorporates an ARM processor, an Ethernet port, and USB ports. The Ethernet port provides connectivity while the USB interface provides access to USB hard drives. Variants on this theme include SATA support. Essentially these systems are NAS (network attached storage) devices.
These NAS devices provide local file services to network attached devices but the idea behind the Plug Computing trend is to provide more services and more connectivity than the typical NAS box. I have done some reviews that take a more detailed look at each of the products but, in general, they work with Internet-based servers to provide remote access as well as services such as backup, transcoding, and multimedia streaming.
Remote access through NAT firewalls is normally accomplished by having a device like a laptop access the vendor's website. The NAS devices are also linked to the website allowing the web server to forward requests between the NAS box and PC. Each NAS device is associated with an account and some services charge for this service in addition to providing other services such as backup.
Much of this can be done with current NAS boxes and NAT gateways but it tends to take an expert to get things right and keep them secure. The services provided by the Plug Computing crowd bring the functionallity to the end user and often the non-computer oriented consumer.
Marvell and most of the vendors such as CloudEngines, Ctera and Tonido provide application programming interfaces (API). These APIs provide access to the device hardware as well as the web-based services. Some even provide app store type services as well.