The U.S. Patent and Trademark Office doesn't allow you to register a trademark if a similar trademark is already used to market the same good or service. The purpose of trademarks is to prevent confusion, and they play a key role in establishing specific brands. This is especially important in sectors of the market that are flooded with similar products.
A registered trademark gives you the right to exclude others from using the trademark for similar goods. If someone uses your registered trademark to sell similar goods or services, you can send him a cease and desist letter or sue him for infringement. Failure to assert your rights can result in you losing the trademark if it becomes generic. "Thermos" is an example of a registered trademark that became generic.
Similar logos are permissible if used to sell very different types of products. For example, the apple logo is used to sell computers made by Apple Inc. and music made by Apple Corps, the famous UK-based music company founded by the Beatles. While these trademarked logos are similar, they attach to dissimilar products. Repeated lawsuits over this issue spanned nearly 20 years. The issue became more complicated when Apple Inc. began selling music online, which was similar to the products sold by Apple Corps. At that point, the parties agreed to settle.
According to Harvard Law School, a distinctive trademark is one that "bears no logical relationship to the underlying product." "Exxon," "Kodak," and "Apple" are good examples of distinctive trademarks. If your trademark is similar to a distinctive trademark, it is more likely to create confusion. Under these circumstances, it may be difficult to defend against a charge of infringement.
The USPTO's Trademark Manual of Examining Procedure emphasizes that the standard for evaluating similar trademarks is whether they create a likelihood of confusion. For example, the TMAP states that "Veuve Clicquot" and "Veuve Clicquot Ponsardin" are likely to cause confusion when used to sell champagne, in part, because "Veuve" is considered very distinctive. Conversely, "Capital City Bank" and "Citibank" are not likely to cause confusion when used to sell banking and financial services.