I have created cartoon characters that I would like to market to
different companies such as Nickolodeon and Disney and wanted to know
how to go about protecting the characters.
Specifically, is it a trademark or a copyright that I'm interested in
(or both), I want to protect the names of the characters as a group,
individually, and their likenesses as well.  How do I do this, with
whom, and how much does it cost?

Hi! Thanks for the question.

The one you need is a copyright. Copyrights and trademarks protect
different things according to the US Patent and Trademark Office.

Copyright:
“Copyright is a form of protection provided to the authors of
“original works of authorship” including literary, dramatic, musical,
artistic, and certain other intellectual works, both published and
unpublished. The 1976 Copyright Act generally gives the owner of
copyright the exclusive right to reproduce the copyrighted work, to
prepare derivative works, to distribute copies or phonorecords of the
copyrighted work, to perform the copyrighted work publicly, or to
display the copyrighted work publicly.”

Trademark:
“A trademark is a word, name, symbol or device which is used in trade
with goods to indicate the source of the goods and to distinguish them
from the goods of others. A servicemark is the same as a trademark
except that it identifies and distinguishes the source of a service
rather than a product. The terms "trademark" and "mark" are commonly
used to refer to both trademarks and servicemarks.”

“What Are Patents, Trademarks, Servicemarks, and Copyrights?”
http://www.uspto.gov/web/offices/pac/doc/general/whatis.htm 

The US Copyright Office of the Library of Congress provides the steps
you need to protect your cartoon characters. Yours will fall under the
Visual Arts category. It mentions Cartoons and Comic strips as
examples of those that can be protected under copyright law. The form
that you must fill up is available here and the fee at the website is
registered at $30.

Visual Arts Registration
http://www.copyright.gov/register/visual.html

The next link pertains to other fees you may have to pay in instances
of renewal or other types of registrations.

Current Fees (July 2002)
http://www.copyright.gov/docs/fees.html 

The following page from the same website provides in detail the
necessities in registering works in cartoons and comic strips.

“Cartoons and Comic Strips”
http://www.copyright.gov/circs/circ44.html 

Search strategy:
Searched in the following websites:
US Copyright Office
US Patent and Trademark Office

I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if you have a question or if
you would need further information.

Thanks for visiting us. Good luck on your characters! Hope to see them
in cable or better yet on national TV in the very near future.

Regards,
Easterangel-ga

Due to changes in US copyright law, anything you make is automatically
copyrighted when you create it, however, you must be able to prove
that you created it. To help in this purpose you would need to
register your copyright with the US copyright office. Last time I
remember the fee was less than $100, but I do not remember exactly
(this is free advice after all). As for trademarks, you cannot
trademark something until you have already been using it in your
business. For instance "always the real thing" could not be
trademarked by coca cola until *after* they started using it in their
commercials. Other than this limitation, you can trademark just about
anything these days.

A good, cheap way to "copyright" something is to mail it to yourself. 
Costs about 37 cents and you just don't open it.  Mail it to yourself
a few times so you have other copies of it.  File them away.  I
believe they are legal in showing the date created.