What is a patent agent?
To represent someone before the USPTO one must be a registered practitioner. Being a lawyer licensed in one or more states is neither necessary or sufficient although most USPTO registered practitioners are also attorneys and are therefore called patent attorneys. A registered practitioner who is not a licensed attorney is called a patent agent. All registered practitioners have equal privileges and responsibilities to the USPTO. They can draft and file patent applications, advise clients on matters of patentability, respond to rejections, file appeals, and otherwise argue before the PTAB (appeals board).
What patent agents do is technically the practice of law, but in a very narrow scope having to do with processes at the USPTO. This question actually went to the Supreme Court in Sperry v Florida in 1963. Florida said it was the practice of law and therefore they could require a Florida law license. SCOTUS agreed that it was the practice of law, but ruled that it was allowed in the state (in its narrow scope) without a state law license due to the constitution, duly enacted federal laws and regulations .
To become a registered patent practitioner you must show to the Office of Enrollment and Discipline that you have the required scientific and technical educational background. This is usually done by submitting a transcript from an undergraduate engineering or science degree program.
After this hurdle is jumped you will be allowed to take the patent bar exam, an all-day test with a 50% first-time pass rate. It is on applicable patent law and patent office procedure, neither of which is logical to an engineer or scientist.
What a patent agent can’t do
Anything that is related to a regular article III court requires a licensed attorney. This would include advice on enforcement of patent rights. Not all attorney patent litigators are registered patent practitioners. They wouldn’t then be patent attorneys but rather IP attorneys.
Trademark Law
Even though the “T” in USPTO stands for trademark, having a USPTO registration does not allow one to practice trademark law. You will need a lawyer if you want help with trademarks, copyright, trade secrets, etc.