Art Commissions? What are they?

[drop.dead.zoe]

I have a Deviantart.com (Deviantart.com/AnimeAddict97). And I have had it for a while. I see people who are holding commissions. And I thought that I new what they were, but I guess I don't. Can anyone give me a good description of them?

 

[Lightmass]

Sweet and simple, it's art that is requested and paid for.