A link uses a word, a phrase, or a design to move a person from one site to another. This is where trademark law comes into play.

A trademark is a word, a phrase, or a drawing intended to identify the source of certain goods. A service mark is a word, a phrase, or a drawing intended to identify the source of certain services. As the "mark" becomes known within a certain market, that mark also begins to stand for a certain quality of those goods or services.

If you're going to see a children's movie, the mark "Disney" means something. It means the movie has something to do with the company that owns the rights to the mark Disney and that the movie will be of a certain level of excellence-- whether it be in the visual aspect of the movie, the music, the characters, or the complimentary merchandise that's usually sold with the release of the movie. Consumers see the mark Disney, then they associate a certain standard with that mark.

There's a good reason that companies react so strongly to someone using their mark without their permission. The purchasing public may confuse the other goods or services and believe the goods/services of both companies are connected in some way. The infringing company could be benefiting by consumers' mistakes as to the true source of the goods/services. Thus, the infringing company is making money off the hard work, expense, and reputation of the true owner of the mark.

For the owner of a famous mark, confusing the public is just one issue. Mere use of the famous mark by someone else may actually dilute the value of the mark. If the infringing company's goods/services are of a lower quality, they could detrimentally affect the reputation of the original company.

By using someone's else's trademark or a service mark as a link on your site, you could be infringing their trademark rights.