Consumers often make their purchasing choices on the basis of recognizable trademarks. For this reason, the main thrust of trademark law is to make sure that trademarks don't overlap in a manner that causes customers to become confused about the source of a product.
If two similar trademarks are being used by companies that provide different products or services, there may not be a trademark conflict. This is especially true if the two businesses serve only local markets and are hundreds of miles apart.
However, in the case of trademarks that have become famous -- for example, McDonald's -- the courts are willing to grant broader protection and prohibit almost all use of the trademark (or anything close to it) by anyone other than the famous mark's owner.
For more information on how to prevent others from using a trademark, see Enforcing Your Trademark Rights.