From the architecture manual:
When designing an OpenStack cloud, it
is important to consider the needs
dictated by the Service Level
Agreement (SLA). This includes the
core services required to maintain
availability of running Compute
service instances, networks, storage,
and additional services running on top
of those resources. These services are
often referred to as the Data Plane
services, and are generally expected
to be available all the time.
The remaining services, responsible
for create, read, update and delete
(CRUD) operations, metering,
monitoring, and so on, are often
referred to as the Control Plane. The
SLA is likely to dictate a lower
uptime requirement for these services.
So, the data plane is everything needed to operate your applications, including instances and their storage and network connecctions. Control plane: Administration of the cloud.