In summary Network Manager is a connection manager and right now supports working with Ethernet/PPP/Wireless.
Network Manager attempts to make networking invisible. When moving into areas you've been before, Network Manager automatically connects to the last network the user chose to connect to. Likewise, when back at the desk, Network Manager will switch to the faster, more reliable wired network connection.
In order to associate to a wireless network and scan NM uses wpa_supplicant as its back-end.
Debugging Network Manager
If you have issues with Network Manager we recommend you start Network Manager with debugging options.