Copyright can be quite a handful — especially when something like theft of your work comes to attention, and ensuring you have a good working knowledge of it is essential in arts practice.
One area that is often mired in doubt, but which has solid foundations in fact, is what happens when you are commissioned to do a work for a client, which often stems from an idea. As stated above, ideas do not come under copyright. But resulting works do. Commissioning holds that it is the client who owns the copyright, not you. However, you will have Moral Rights to the work e.g. being correctly named/credited or identified as the person who created the work (applied as Moral Attribution). Many artists run into strife entering exhibitions, festivals and displays that insist on part of their "Terms and Conditions" on ownership of Copyright. That cannot be done without a formal Reassignment of Copyright — handing over your work to another person (usually for an obscene amount of money, the more the better), which in effect means he/she can call the work you created his/her own. Intellectual fraud is probably a better way of putting it, though a lot of money can be made.
In Australia Arts-Law degree courses deal with copyright in some depth (foundation law, contract, moral, intellectual, performance, visual, literary etc., of which have some very important tenets of note. The internet — a bastion of relatively existential lawlessness, has stretched the boundaries of who owns what (and how to prove it), and how, when and where it should be displayed and this is an area that keeps many lawyers in Arts practice very busy. Simply, if you don't want your work stolen, don't put it up on the internet.