The fourth type of intellectual property is the least well known (the first three: copyright, trademark, and patent). Trade secrets don't have a government office that registers them, because they're protected by being kept secret. This page from the USPTO has a brief introduction, and don't forget that trade secrets were a topic of this blog a couple of years ago.
Here in St. Louis, this young entrepreneur probably won't tell you how he makes his car air fresheners, because it's a trade secret. He has clearly indicated that he is using Mr. Fresh name as a trademark by using the TM symbol, and the bottom of his webpages show that he intends to maintain his copyright in their creative expression. Right there, is a great example of three of the four US types of intellectual property protection.
Got more questions about intellectual property, or have a fun example to share? Contact the Patent & Trademark Resource Center to share or ask.