FL(Flight Level) vs Altitude
This depends on where you are in the world but under the FAA here in the US the FAA defines it as 


  Flight level means a level of constant atmospheric pressure related to
  a reference datum of 29.92 inches of mercury. Each is stated in three
  digits that represent hundreds of feet. For example, flight level 250
  represents a barometric altimeter indication of 25,000 feet; flight
  level 255, an indication of 25,500 feet.


In other words its the altitude if you set your altimeter to 29.92. In the US all aircraft in class A airspace (18,000 and up generally) set their altimeter to 29.92 thus flight levels are used at that altitude. When an aircraft is below Fl 180 the local altimeter setting is used.