A trademark is a word, name, symbol, device, or combination of, used by someone to identify his product. Trademarks arise from ‘use’ and do not have to be registered to be considered trademarked. There are good reasons to register a trademark though. One reason, like copyrights, it establishes a public record. The second reason is that it needs to be registered in order to file for trademark infringement. It also helps to establish trademark in other countries and to stop imports of infringing foreign goods from entering the country. A trademark is valid indefinitely, but if not maintained it can be lost and fall into public domain. For instance, if a trademark becomes a common phrase, then it will be deemed lost and the trademarked term considered common usage (Aspirin, Allen Wrench, Granola, and Yo-Yo are just a few examples).
Trademark registration begins with the U.S. Patent and Trademark Office (P.T.O.). Registering a trademark can take more than a year after the application is filed. There is an extensive research involved to ensure that a similar trademark does not already exist.
Once the trademark goes through, the ® symbol identifies a trademark as registered with the U.S. P.T.O. The proper way to write this is – “® Registered in the U.S. Patent and Trademark Office”, or the abbreviation – “Reg. U.S. Pat. and Tm. Off.” If it is not yet officially registered with the P.T.O., the ™ symbol should be used instead.
Trademarks are protected from infringement and also dilution. Infringement of a trademark means that there is another that is too similar and it is confusing. Dilution of a mark would be because the public has a strong association with the original trademark and the other would take away from that association.
It is not considered infringement to make fun of a copyrighted or trademarked work as long as it is apparent that it is not the original, but a parody. You can not create a domain name similar to another and make fun of it, because it would not be evident that it was a joke until the user actually reached the website.
Trademarks should not be used in meta-tags (the hidden keyword tags on a web page), or in a pay-per click ad campaign. There have been cases where this was considered infringement.