OpenStack is a cloud operating system that controls large pool of compute, storage and networking resources throughout the data center. OpenStack provides GUI interface known as dashboard to manage and control the resources for its users.
OPNFV stands for "Open platform Network Function Virtualization" and it's focus is to enhance NFV. OPNFV is a platform which uses other open source components such as OpenStack to get Iaas (Infrastructure as a service ) and introduced/replaced other existing component to the open source. For example: OPNFV uses opendaylight for networking related things instead of Neutron of OpenStack.
This is what I know. Please correct me if I stated wrongly.