Pilot Licenses and Ratings
This is a clear description of Airplane Pilot Licenses - what they are and how to get them. Actually they're called Pilot Certificates. A license grants a permission, whereas a certificate shows that one has fulfilled certain requirements.
The Certificate Grade is the hardest one to change. In order to get a new Grade Certificate you need to meet all the training and experience requirements for that certificate. The process is called upgrading and requires you to have to have the certificate with a lower grade. Student Pilots and Recreational Pilots can upgrade to Private Pilot. Only Private Pilots can upgrade to Commercial Pilot. Finally only Commercial Pilots can upgrade to Airline Transport Pilot. You are always required to take a knowledge test and a practical test in order to upgrade.
Adding Ratings within the same aircraft category is significantly easier. Except for the initial instrument rating, there are no knowledge tests or extra aeronautical experience requirements. An instructor endorsement and a practical test is all it takes, and yes - there is some textbook study for the practical test.