The private pilot license allows a pilot to operate a SEL aircraft under normal VFR conditions. It is the certification held by the majority of pilots and the first step to becoming a commercial pilot.
After obtaining your private pilot license, the next step is to complete the instrument certification. This certification allows a pilot to fly with decreased visibility and lower ceilings.
The commercial pilot license allows a pilot to receive compensation for flying. The requirements for the commercial license include several hours of training in a complex aircraft.
The Multi-Engine license is an "add-on" to the single-engine license and allows a pilot to fly an aircraft with more than one engine. This license broadens a pilot's horizon in regards to speed and power.
There are several options for pilots to hour-build, but one of the common ways is for the pilot to become a CFI. A certified flight instructor builds his hours while training future aspiring pilots.
CFII & MEI
Advanced and Multi-Engine Training
The CFII is the next step after completing the CFI training. The CFII allows a flight instructor to instruct instrument students and an MEI allows an instructor to give his students multi-engine instruction.