Neutron is a virtual network service for Openstack. It is an OpenStack project to provide "networking as a service" between interface devices (e.g., vNICs) managed by other Openstack services (e.g., nova).
Neutron mission is to implement services and associated libraries to provide on-demand, scalable, and technology-agnostic network abstraction. Like Nova Networking, Neutron manages software-defined networking for your OpenStack installation. However, unlike Nova Networking, you can configure Neutron for advanced virtual network topologies, such as per-tenant private networks and more.
Starting in the Folsom release, Neutron is a core and supported part of the OpenStack platform. It was previously known as 'Quantum'.