Distributed Resource Scheduler - handles initial vm to host placement and initiates vMotion based on host CPU and host memory constraints.
Enterprise: Storage DRS - n/a
Standard: DRS - n/a, Storage DRS - n/a

In vSphere, vSphere DRS can configure DRS affinity rules, which help maintain the placement of virtual machines on hosts within a cluster. Various rules can be configured. One such rule, a virtual machine-virtual machine affinity rule, specifies whether selected virtual machines should be kept together on the same host or kept on separate hosts. A rule that keeps selected virtual machines on separate hosts is called a virtual machine-virtual machine anti-affinity rule and is typically used to manage the placement of virtual machines for availability purposes.

Also new with 5.5 is the ability to live storage migrate vms protected by vSphere replication (or automate it with DRS)

Configuration is simple (tick box) and can be set to manual (recommendations only), partially automated (automatic placement) or fully automated.
With vSphere 5.1 Storage DRS is interoperable with vCloud Director.
vSphere 5 introduced Storage DRS - the ability to logically pool storage resources (datastore cluster) and migrate the actual virtual machine data to other disk resources based on performance criteria (i/o and latency). Storage DRS makes initial vmdk placement and gives migration recommendations to avoid I/O and space utilization bottlenecks on the datastores in the cluster. The migration is performed using (live) storage vMotion.