It is a C-40, the US Air Force version of the Boeing Business Jet (BBJ).They are in the process of buying a handful to update their transport fleet used for Special Air Mission (SAM) flights. These are any flights carrying US Government or Diplomatic staff.
Technically when the President flies on one of the fleet this is a SAM flight, but as already stated any aircraft with him on board will take the callsign "Air Force One"
Regards,
PHX