*The basics are clear, if you create brand confusion by using a competitor's trademark (without a disclaimer that you are not the competitor) then you have violated the trademark. If you are selling apples, you can use the word "apple" but if you are selling computers, then using "apple" would not be advised. However, some people choose to make a little disclaimer on their site which reads something like this "ABC Company is not affiliated with Apple Computer" with the hope that the words "Apple Computer" would then get picked up as an anchor. It's tricky and legal. The part that is unclear is this; if you are using the brand name of a competitor in an effort to solicit their customers, like choosing their brand name as a key word in Google adwords, then the world has mixed feelings. Recently Google made the policy very clear. They lifted the restriction on using competitor keywords, citing the legal opinion that use of a trademark to seek out and solicit customers is perfectly OK, so long as the competitor's keyword does not appear in your ad copy, where you might confuse the customer as to what brand they are buying. (*This is not legal advice.)