The law of trade marks comprises one of the branches of the law of intellectual property. Unlike copyrights and patents, trade marks do not protect the implementation of an idea by the way of patentable invention, nor actual creative expression by way of copyright, but protect product reputation and goodwill. One of the other differentiating features of trade marks from patents and copyright is that if they are not enforced (leaving minor or inconsequential misuses out of account), they can lapse. This means that those who own valuable marks must be vigilant about whether others are misusing their marks in a consequential way, and enforcing the marks if they are.
A registered trade mark is an expression or symbol which identifies particular products or services as being those of a particular trader. The breach of a trade mark arises if another trader describes one of their own products or services in a way which might confuse it, in the eyes of a reasonable person, with those of the trader protected by the mark. If the trade mark is registered, this gives a separate right of action apart from the common law wrong of "passing off": the misusing of the mark becomes a cause of action of itself in the UK (including Scotland). US trade mark law is similar to that in the UK and many other countries, although in the US there is the additional concept of "trade mark dilution": that use of the mark by others might devalue it even if no product confusion results.
However vigilance can be taken too far. In the US there is a National Pork Board, an organisation representing those producing and trading in pork meat. The Board has a registered trade mark in the expression "The Other White Meat", which is widely used in the Board's advertising (presumably this is an attempt to make pork appeal to turkey-eaters).
ThinkGeek are an internet trading organisation selling weird products on the internet that most people wouldn't buy to save their lives, but that some people apparently like (I must confess that the Monty Python Killer Rabbit slippers look quite appealing). It is tempting to think that the whole site is a spoof but that is apparently not the case. However they do from time to time do spoofs, and on April 1st this year they advertised their unicorn meat product, under the buy-line "Pâté is passé. Unicorn - the new white meat". Apparently someone at the National Pork Board, zealously protecting their trade mark as required by law, thought that unicorns really existed, and that the use of the expression "the new white meat" might cause a reasonable consumer to confuse the product with pork, notwithstanding the sales information which included:
"Excellent source of sparkles!The Board went as far as issuing a "cease and desist" letter via their lawyers. This must be one of the epic legal blunders of modern times.
"Unicorns, as we all know, frolic all over the world, pooping rainbows and marshmallows wherever they go. What you don't know is that when unicorns reach the end of their lifespan, they are drawn to County Meath, Ireland. The Sisters at Radiant Farms have dedicated their lives to nursing these elegant creatures through their final days. Taking a cue from the Kobe beef industry, they massage each unicorn's coat with Guinness daily and fatten them on a diet comprised entirely of candy corn."
Suffice to say, the spoof "product" remains advertised by ThinkGeek.