Professional...one who's sole income is made by doing some job or activity where a qualification or degree of skill or expertise is required.
Semi professional...one who gets paid for a job or activity but does not rely on this as sole income.
Amateur...one who participates solely for enjoyment without wanting or receiving payment.
Hobbyist is a word that has come to mean an enthusiast or someone who pays to participate. They generally buy top of the range pro gear.....sometimes these people are seen as a cash cow by equipment suppliers and scam merchants.

It is worth repeating that a high degree of talent plus a determination and a hard work ethic or even a complete lack of any of these qualities can be demonstrated by anyone from the above categories.

Some guys who have dedicated their whole life to music and demonstrate exceptional talent go unnoticed whilst some tone deaf teenagers who wander in off the street to X factor receive great acclaim and can make obscene amounts of money. I am sorry but IMO the true meaning of Professionalism, talent, stardom, fame and fortune has gone out the window.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055