“Common law trademark rights” are rights which arise from use, e.g. selling a product under a trademark or providing a service under a service mark. They are not governed by statute. Common law rights are developed by the courts and governed by state law. Common law trademark and trade names are non-statutory, use-related rights.
These rights exist in addition to state & federal statutory rights obtained through federal registration. As soon as a product using a trademark is sold, or when services are performed under a service mark, or when a business is operated under a trade name, the owner or seller acquires common-law rights that can prevent others from using the same or confusingly similar marks or names, for similar goods/services or businesses in the same market territory.
Federal registration of trademark provides many benefits that common-law rights do not provide.