What is the difference between "flight level" and "altitude"?
Flight levels use QNE or pressure altitude, while altitude references QNH or local pressure adjusted to sea level pressure.  Altitudes are used at low levels and flight levels at higher levels.  The transition between altitudes and flight levels differs by country and is generally just above the highest obstacle in that country.  In the US the transition altitude/level is 18,000' / FL180.  Some countries transition as low as 5000' / FL050 and the transition altitude/level may vary from airport to airport.  

In the altitudes knowing accurate elevations relative to the ground and obstacles is important for collision avoidance and this is the reason QNH is used here.  Each airport will report QNH and controllers will issue the current QNH as needed.  You need to know the QNH for obstacle / terrain avoidance but you need to be using the same QNH as those around you for aircraft vertical separation.

Above all terrain/obstacles the only thing we care about is vertical separation, so we no longer need to know about the actual pressure and instead use a standard reference pressure, QNE / 1013.25 hPa / 29.92" Hg.  

Note that flight levels drop the last two zeros of the corresponding altitude and so 30,000 is FL300, not FL30000.  When checking in with a controller, FL300 would be pronounced flight level tree zero zero. 

It is also worth noting that an altimeter cannot actually determine altitude.  It can only determine pressure (technically local static pressure compared to a reference pressure).  It converts this pressure to an altitude using a calibrated non-linear scale.  To illustrate the point, look at this map of 500 mb heights:



500 mb correlates to 5500 m or 18,000 ft in a standard atmosphere.  In a real atmosphere this height varies and is not actually level.  An airplane flying "level" at FL180 from LAX to NYC last night will have actually descended almost 300 m while indicating a constant altitude.  These deviations in true altitude from indicated altitude are acceptable, however, since they effect everyone equally in the same locality and separation is maintained.