(I have researched this, but I am not a lawyer. My references come from the U.S. Patent and Trademark Office (http://www.uspto.gov), but any interpretation of them by me is just that--my interpretation. I encourage you to, after reading this, do your own research.)

First--In most cases, the average webcomicer or other online content provider does not need a trademark. There is nothing wrong with having one, but read on to determine whether you need one and/or whether you need to register an existing one (the short answer to the registration question being no).

What to trademark
For most practical purposes, the bulk of the things a webcomicer is concerned about protecting are actually covered in a roundabout way under copyright law (covered in a separate article). Still, you may have some catch-phrases or other things you would like to tradmark, and it’s good to know a little something about trademarks in general. This article should help dispel some of the myths about what they are and what they do.

Trademarks (Huh! Good Gawd, ya'll)--What are they good for? (Don't worry if you don't get the reference.)
According to the USPTO, a trademark is, "a word, phrase, symbol or design, or a combination of words, phrases, symbols or designs, that identifies and distinguishes the source of the goods of one party from those of others." (http://www.uspto.gov/web/offices/tac/doc/basic/trade_defin.htm)

Notice that phrase, "identifies and distinguishes the source of the goods..." Correlate that phrase with the information at http://www.uspto.gov/web/offices/tac/doc/basic/appcontent.htm#basis, and you see that a trademark is, essentially, a way of ensuring that goods or services are indeed coming from where they look as though they're coming.

Trademarking something doesn't mean that no one can use it at all, though--Just not in such a way that it might confuse a customer about the source of the goods or services.

Here's an example of trademarking--Marvel and DC filed a joint trademark on the word, "superhero," in all it's forms. It was established in court that most people, when they think of superheroes, think of Marvel and DC. Does that mean you can't call your characters superheroes? Not at all!

Despite the huge outcry about the trademarking of the word, most of it was due to a misunderstanding. You can still legally use the term superhero in all its variations as much and as often as you want in your comic. You can have characters call themselves, and be called, superheroes, and have the narrator (if any) refer to them as such.

What you may not do is make the word a part of your title or subtitle, or use it in any other way that might lead a consumer to think that he or she is buying a product of DC, Marvel, or a company owned by either of them, or that they endorse your product.

You cannot, for example, sell a shirt with a picture of your character and the caption, "Chicago's number one super hero!" Nor could you market a cereal as "Superhero Flakes." Both of those would imply at the very least a licensing deal with DC and/or Marvel, and could give the impression that one or both of them endorse your product.

Here's another example. Say you've developed a distinctive logo for your site--which is your product. You've been using it for a couple of years now, and you've put the little TM thingie on it. One day Joe comes along and starts using it on his own site. Well, this could obviously confuse people about whose site they're at, so that's violation of your trademark. On the other hand, if Joe just shows one of his characters wearing a T-shirt with your logo on it, he's probably within the law.

Yet another example, many, many sprite comics are in violation of trademark law for using trademarked names in their titles, etc. For instance, a webcomic called, "Sonic and the Mario Brothers," could be found to violate trademark law, as it could be interpreted to imply a connection between the producers of the game and the comic.

One final example—I’ve spoken to a few people who were worried about using brand names in their comics and other fiction. In most cases you have nothing to worry about. Remember that the issue here is giving the impression that your work is somehow affiliated with, manufactured by, or endorsed by the owner of the trademark, so yes, it’s okay for your character to order a Coke and drink from a can with the Coca Cola label.

That’s good news, too. Who among us has time to come up with an entire world’s worth of company names, product names, and logos? Certainly not me, and probably not you. So yeah—Go ahead and have your characters accidentally level the local Taco Bell during a super-powered battle, drive a Jeep Cherokee, or order Glenlivet neat.

Now, just staying within the bounds of trademark law doesn't mean you are off the hook about everything. One place you could still run into trouble here would be if you started publishing nasty things about a company--even if you're not technically in violation of trademark law, and even if your defense id, "but it's fiction." For instance, if you need an evil corporation to try to take over the world, don’t use the name Microsoft, however close to reality you feel that may be. Don’t have your psycho killer poisoning the Gatoraid supply for the American Northwest. A little common sense here goes a long way. Imagine yourself the head of a major corporation and think about what kinds of portrayal of your company would get your dander up, and just how much time, money, and expense that you, if you were that CEO, would be willing to expend to put an end to it.

In other words, do not meddle with the reputations of big corporations with huge legal teams, for you are small and crunchy, and taste good with ketchup.

Trademarks--Do they have to be registered? (U.
Despite all the misinformation out there, the answer is no, so you can save your three hundred bucks.

According to the United States Patent and Trademark Office, "You can establish rights in a mark based on legitimate use of the mark." (http://www.uspto.gov/web/offices/tac/doc/basic/register.htm)

The site lists several advantages to registering a trademark with the federal government, but is unclear on what rights one has with an unregistered trademark.

The page seems to imply that one can take advantage of most of the rights of a registered trademark holder even if unregistered, but that being registered establishes your right to the mark beyond question--as in if you have to go to court, the issue of whether you own the trademark is already established, and all that is up for consideration is if it is being violated. If it has not been registered, you would presumably be required to prove that you have established rights to it. (I will get confirmation or denial of this and update this tutorial at that time.)

Basically, though, feel free to use the TM (trademark) wherever appropriate—The Trademark and Patent office is very clear about this. You may not, however, use the R mark unless you have registered your trademark.