Please take a look at the following questions:
- What is NSX?
- What the heck is the difference to Nicira NVP or are they the same?
- What are the technologies behind NSX and how does it work?
Is there any you cannot answer, yet? If so, I would like to direct your attention to two just great videos on Youtube which will clarify:
OpenStack Networking – Theory Session, Part 1
OpenStack Networking – Theory Session, Part 2
Watching this will be the best 1h 45min you have invested for a while!