From How to Self Publish Comics ... Not Just Create Them, by Devil's Due Publishing, 2006.
If you own the copyright to something, it means, basically, that you have the right to copy it, to reproduce it in any medium. For example, you've created a character named Jimmy AllPowerful, and asked me to draw a picture of him. I agree to do so for free, and tell you that you may publish it as a pin-up in one of your comic books. Although you created the character, I own the copyright on that image. If you want to use that image in another comic book, or on a T-shirt, you must again ask me for permission. If you're selling a lot of comics, I'm probably going to want some money.
Notice: I still do not own the character, just the image I drew of it. That's because you own the trademark. If I took that image, and produced T-shirts of it without your permission, you would have just cause to make me stop (or as they love to say, "cease and desist").
Most states have laws that automatically protect your creations the very moment you put pen to paper, or fingers to keyboard. However, it never hurts to mount the evidence in your favor, so it's a good idea to officially copyright your work through the government.
The symbol used to identify copyright ownership is ©. To type this on your computer, simply type "alt G". Contrary to popular belief, you do not have to register your art or text with the government to place a copyright symbol on it. Registration with the state or federal government simply helps strengthen your claim of ownership in court.
Trademark is the ownership of more than just an image: it's basically the ownership of an idea. It is the general concept, and all of the associated characteristics that go along with it. It's really hard to explain, but once you understand, the concept of "trademark" should be very simple.
The US Government breaks trademarks into categories and various media. For instance, Superman is a term that's been around a lot longer than the DC comic book everyone knows today. The concept of a hero in tights and a cape is also a very general concept that no one person can claim to own. Krypton is an actual element on the periodic table of elements. Lastly, the letter "S" is simply a part of the alphabet, and free for everyone to use. Put them all together, though, and you have a trademark.
DC's Superman is a powerful hero from the planet Krypton, wearing blue and red tights and a cape, with a very unique "S" design on his chest - and these are some of the details that comprise the trademark of Superman. Even more specifically, this trademark applies to comic books. Now, Superman has been around for a zillion years, and has been merchandised into many other media, and DC (or their parent company Warner Bros.) has taken care to register the "TM" in various categories. You still hear the term "Superman" in songs, though, and there's nothing DC can do about it unless the song refers to very specific elements of their character's trademark.
The universal symbol for Trademark is simply, "TM," seen somewhere on most logos and product packaging. Just like copyrights, if you have created something, you have every right to place a "TM" by its name.
This is the big daddy of copyright and trademark law. If you see any product with an ® ("alt R" on your keyboard), that means the owner has officially registered it with the Library of Congress. They've sent images, text descriptions, and logos to the Feds, and paid a few hundred bucks to do so, to ensure they protect their property as best as possible.
Again, I must reiterate that I am not an attorney, and you should seek professional advice on the subject, but this gives us a start."
So, pretty good advice on copyrighting your creations, I think!