An FAA Private Pilot License requires theoretical and practical training to operate the airplane safely which authorizes the holder of the license to act as Pilot in Command of a civil aircraft.

An FAA Instrument Rating gives the holder of a Private or Commercial License the additional privilege of flying IFR (Instrument Flight Rules). 

An FAA Commercial Pilot License allows the holder to fly for compensation or hire. 

An FAA Flight Instructor License allows the holder to teach other pilot trainees how to fly. 

A CFII License allows the holder to teach other pilot trainees how to fly under instrument condition, also known as Instrument Meteorological Conditions (IMC).

A Commercial Multi-Engine Add-On rating allows you to practice commercial pilot privileges in a multi engine airplane.