Q: What kinds of copyright are there?
A: Let's have a look at a few definitions, remembering I am not a lawyer, but a fellow freelancing artist-illustrator trying to learn about these things. Despite the fact that it can contain inaccuracies, I will be linking to Wikipedia a lot in this Going Pro: I suggest confirming information with the useful citations and links found at the bottom of the entries.
1) Intellectual property
Most types of copyright fall under intellectual property laws. You'll often hear creative types discussing protecting their IP. What they generally mean, is it's not enough to protect the idea, you have to protect the execution of that idea. Intellectual property as a concept deals with the intangibles of the idea in the mind, and tangibles or artistic creation.
2) Copyright
Copyright refers to the legal right of a creator over their creations. You don't have to register to have copyright (though there may be good reasons to register, covered below). By creating a piece of art, you are now a copyright holder in most countries under the Berne Convention. Congratulations!
Copyrights are usually indicated by the © symbol. You can easily type this into your work in Photoshop or even blogging platforms by typing "[hold down alt]-0169".
3) Moral copyright
Moral copyrights refer to the right to have artwork credited to the creator, even if the economic rights to use the artwork is sold to a client. In some cases, moral rights can also extend to not altering the work.
In an important case in the city of Toronto, Canada, artist Michael Snow had been commissioned by the Toronto Eaton Centre to create an installation sculpture of a flock of Canadian geese, titled Flightstop (right).
(Good info on Wikipedia, Jan 2010.) The Eaton Centre placed red bows on the geese for the Christmas holidays, and the artist successfully sued, contesting that they did not have the moral right to alter his artwork, despite that they had paid for it. This is an important example to artists, and I would suggest having a mention of moral rights in any contract. The result of suing or contesting a disagreement may not always be for money, unless damage to the artists' reputation has occurred. You may be only seeking legal action to restore the artwork to its original state. As a thought experiment (I know of no such examples), what if you were commissioned to paint a barosaurus, and later discovered the client lied about their affiliations with their institution and were now using your work prominently to promote creationism? That may damage your chances to get future work within the scientific community and you would need to redress what has happened to your moral rights.
4) Trademark and registered trademark
Trademarks are a way of identifying an image or product as representing a badge or symbol of a company. You may be a freelancer who incorporates their job, and uses a small symbol to denote your art - that could be your trademark. It's basically a way of identifying your brand, and trademarks refer to a brand.
If your trademark has been infringed, you can sue, but it may be difficult to do so out of a legal jurisdiction to which it originated.
Trademarks are generally noted by the ™ symbol. There's a similar symbol, ℠ for services rather than goods.
Registered Trademarks are usually allowed if you can prove your brand is distinct from all others, and register it with an agency, such as the U.S. Patent & Trademark Office. Individual countries will have similar agencies.
Registered trademarks are noted with the ® symbol.
5) Patent
Patents generally refer to created or invented goods, so are not likely to be relevant to illustrators here at Art Evolved. Unless you invent a spiffy new brush for doing delicate scale work and wish to market it, patents don't need to be covered too much.
If you do create a patent, you may want to register it to your country's patent office. 
Interestingly, the Danish company Lego has taken legal action in a number of countries against upstart Canadian company MegaBloks. In most cases, Lego has failed, due to the shape of their bricks belonging to patent law and not trademark law, and the patents have expired in many countries. In Canada, I believe the decision was that you could not trademark or patent a geometric shape. Picture at right shows a MegaBlok on the top, and Lego brick on the bottom.
6) Fair-use
So say you are sitting in a lecture at a university about dromeosaurs. And lo and behold, a piece of your artwork appears on the screen, illustrating a species' long running legs. You haven't granted any use to the professor: what's going on? Fair-use is an exception to copyright, allowing educational institutions to skirt the necessity of paying or procuring the right to use artwork for the sake of a single example, in lectures or talks. It may also come into play in journalism, such as a magazine article covering your art show and including a photograph.
Fair use can be a bit of a grey area, and in most districts, something like re-printing your artwork on a handout for a class is still considered a violation of your copyright.
7) Transferred copyright and licencing
In some cases, a copyright can be transferred. When this occurs, the client may ask the artist for unlimited use of their tyrannosaur cartoon for mugs, t-shirts, stuffed toys and so on. Typically, artists charge up to a few hundred percent more than their usual fee before surrendering unlimited, transferred copyright to a client (I would suggest looking at the Graphic Artists Guild Handbook to get an idea of what you should charge - great book).
In some cases, an artist may be hired to make all of their drawings while working for a company. Tim Burton created his Nightmare Before Christmas characters while under a contract to Disney: when he wanted to make the movie, he did it with Disney's Touchstone label, since they had rights to the works. Similarly, Barbie-rival Bratz is alleged to have been created while the Bratz creator worked for Mattel, so Barbie may effectively own the Bratz.
A common licencing arrangement artists may want to consider in any contract, is to grant unlimited rights for a specified time period. If you create that tyrannosaur for unlimited rights, you may want to specify it as only being for the next two years, after which the client's use expires or needs to be re-negotiated. This is useful in case the image you've made becomes a runaway hit, and gives you another shot at further monetary compensations, such as royalties.
In most cases, I would not advise my fellow Art Evolved illustrators to transfer their rights, or grant unlimited licences. It's generally better to work use by use in my experience.
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I hope this has been a useful reference, and thanks for joining me for another Going Pro here on Art Evolved. Cue the theme music.
