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.