For Your Information!

.: Requests :.
When someone asks the artist to do art for them for free.

.: Commissions :.
When someone pays the artist with money (Paypal or mail) to do art for them.

.: Collaborations :.
When two or more people work on one piece of art (e.g. one person draws and the other colors).

.: Trades :.
When an artist does art for someone and that person(usually another artist) does art(or something else) for them in return.