Trademarks apply with a scope.
For example, you can have a "McDonald's" hardware store.
Some trademarks and service marks also include the styling (font) of the word. So Apple can certainly trademark their name as a logo with a particular font.
Amazon is another obvious one.
According to the trademark law, trademark word candidates are divided into five categories, from (a) fanciful words - strong trademarks, to (e) generic terms - not protectable at all. The stronger the mark, the more protection it will be given against other marks. The categories, ranked in decreasing order in terms of strength, are:
(a) Fanciful Marks—coined (made-up) words that have no relation to the goods being described (e.g., EXXON for petroleum products).
(b) Arbitrary Marks—existing words that contribute no meaning to the goods being described (e.g., APPLE for computers).
(c) Suggestive Marks—words that suggest meaning or relation but that do not describe the goods themselves (e.g., COPPERTONE for suntan lotion).
(d) Descriptive Marks—marks that describe either the goods or a characteristic of the goods. Often it is very difficult to enforce trademark rights for descriptive marks unless the mark has acquired a secondary meaning (e.g., SHOELAND for a shoe store).
(e) Generic Terms—words that are the accepted and recognized description of a class of goods or services (e.g., computer software, facial tissue).
My understanding is that sometimes words become "famous marks" because of widespread use. Apple is a good example...very, very unlikely they would issue a trademark for that word even in a completely different market....because the word Apple is so widespread in society.
One trick is to trademark the word with .com at the end....and always use it that way...
Amazon trademarked "Amazon.com".