Other.law.Trademarks

DISCLAIMER: I am not a lawyer. This is an overview and should not and may not be used as legal advice or a legal authority.

The Short Version:
A trademark uniquely identifies one product or company in context of the trademarks' use. The purpose of trademarks is eliminate confusion in the market place. So, for example, I couldn't make a software program called Flex because Flex is a trademark of Adobe. However, I could create an exercise video and call it Flex because there is no way the two could be confused in context.

What will cause confusion is a subjective. Microsoft claimed that "Lindows" infringed on the "Windows" trademark because users would think, solely based on the name, that "Lindows" was made by the same company as Windows. However, I could open a store "My Corner Store" in Philadelphia and trademark the name despite the fact that someone had already created a store with the exact same name in San Fransisco because it is unlikely that the two will be confused.

Trademarks need not be words. A trademark can be anything used for identification. Blogger's logo in the top left corner of this page is a trademark. Sounds, colors and motion can be trademarked (although these are less common and harder to enfocre).

Lastly, trademarks do not expire, but they do dissolve when they stop functioning as a unique identifier. The most famous example is Aspirin which used to be a specific drug made by one company. However, people started referring to the generic form of Aspirin as Aspirin and eventually, in the public's mind, the word Aspirin referred to either the brand name or the generic version of the drug. Thus the trademark on Aspirin was lost because it no longer unquietly identified as specific product.

While I would never endorse Wikipedia as an authoritative source, their article is a good starting point for more complete information.