In most cases, your so-called “common law” trademark rights start when you start using your trademark “in commerce” in connection with your goods & services. This does not require a special filing at all. (The “common law” is the state-specific legal framework that has been created by courts and is not governed by federal statutes). The Lanham Act (15 U.S.C. § 1125(a)) provides that you can sue under these common law rights in federal court as well.
A federal trademark registration gets you a slew of additional protections such as:
Presumptive rights that your trademark is not descriptive
Presumptive rights to assert your trademark across the United States (as compared to an unregistered trademark that only enables you to assert your trademark in states where you use the mark).
Public notice of your claim of ownership of the mark;
Listing in the USPTO’s online databases;
The ability to record the U.S. registration with U.S. Customs and Border Protection to prevent importation of infringing foreign goods;
The right to use the federal registration symbol “” (as opposed to “TM” or “SM”)
The use of the U.S. registration as a basis to obtain registration in foreign countries