When explaining trademarks to clients or potential clients, I like to use the “swiss cheese analogy.” In general, I consider trademark registrations to be like a piece of cheddar cheese. A Federal Patent & Trademark Office trademark registration gives the owner the exclusive right to use that particular word or design (or sometimes, color, sound, or even smell) throughout the United States in connection with certain narrowly defined goods and services noted in the application. So the registration essentially covers the entire map of the United States.
However, if the trademark owner has not properly cleared the mark for registration and use, there could be “common law” users lurking and the piece of cheddar cheese can start to look like a piece of Swiss cheese. Common law (unregistered) users may have been using the same or a similar mark for the same or similar good and services, before the registrant. These common law users have prior rights within the geographic area where they’ve been using the mark, poking holes into the owner’s Federal registration.