The first thing to understand is that the name is a bit of a misnomer. VSAN has got nothing to do with SAN in the traditional sense. Instead it leverages the local storage from a number of ESXi hosts which are part of a cluster. A distributed vsanDatastore is then created leveraging the local storage from each of the ESXi hosts. This can then be used for VM placement, and of course supports a range of core vSphere technologies like vMotion, DRS & vSphere HA.
Virtual SAN is fully integrated with vSphere. It is an object based storage system and a platform for VM Storage Policies that aims to simplify virtual machine storage placement decisions for vSphere administrators. Its goal is to provides both high availability as well as scale-out storage functionality. It can also be thought of in the context of quality of service (QoS) in so far as VM Storage Policies can be created which defined the level of performance and availability required on a per virtual machine basis.
VSAN can be though of as both a converged platform (of both compute and storage) as well as hybrid storage solution (since it leverages both SSDs and traditional spinning disks).
The scale-out aspect is not to be overlooked. If you need additional storage, additional storage can be added to the hosts and automatically added to the vsanDatastore (which may be configured to increase its capacity on the fly). Should additional compute be needed, then it is simply a matter of dropping a new ESXi host into the cluster. Any storage on this new ESXi host may also be claimed by VSAN if configured to do so.
And there are no additional VIBs or appliances to deploy either. VSAN is baked completely into vSphere 5.5. All components required to create a VSAN product are built into ESXi and vCenter server (both the Windows and the appliance version).
Needless to say, this is something very exciting for us all here at VMware. While we have dipped our toes into the storage space in the past with the vSphere Storage Appliance (VSA), VSAN is a different product entirely.
Last but not least, why not get involved in the VSAN beta. All the details are here, and it will provide you with an opportunity to provide direct feedback on VSAN (plus there are a couple of prizes for the first sign-ups):