VSAN is a new Vmware developed storage solution so VSAN is used to store your Virtual machines disk files .
VSAN if fully integrated into Vmware vSphere in fact it’s a part of vSphere hypervisor .
Vmware designed VSAN as a platform for policy based storage to simplify your virtual machine deployment decisions.
what this means is by calling local storage and integrated it into the hypervisor in Vmware Vcenter for management you could actually create storage polices for each of your different virtual machines to insure that does virtual machines are being given the storage quality of services for example that the applications running inside require .
Vmware VSAN is also a highly available clustered storage solution and Of course since it is a Vmware product the majority of Vshere advanced features things like vMotions , storage vMotions , DRS , HA and so on are fully supported when run on top of the VSAN shared data store.
Vmware VSAN is also scale out storage system so what that means is as that your virtual machines requremnts grew means you needs more storage capacity you can simply add more storage to your existing servers or you can even add new servers or host to the VSAN cluster which also brings additional storage capacity.
and finally i want to point out that VSAN provides different Quality of Service (QoS) levels for each virtual machine or each virtual disk.