Having done this many times, let me answer.
When checking in at LHR, your bag will receive a tag indicating that BOS is is final destination, but you will need to clear it through customs at EWR.
You will arrive in EWR either at Terminal B or C. After leaving the plane, you will first go through immigration (inspect passport, visa, fingerprint, photo, etc.). You will then enter the baggage hall where you will pick up your baggage from the belt and carry it through a customs inspection. After that, you will be in the recheck area where you will return your bag to United for transfer to your Boston flight. You must then check the video monitor to determine whether your BOS flight leaves from Terminal C or A.
Arrive at C and depart from C: you follow the signs in C to security, clear security and go to your gate.
Arrive C and depart from A: follow the signs to the Airtrain, take it to A, clear security in A and go to your gate.
Arrive B and depart from either A or C: follow the signs to the Airtrain, take it to A or C, clear security in A or C, and go to your gate.
Once you arrive in BOS, you saunter off the plane and to the baggage claim area to collect (hopefully) your bag. You then exit the airport for your taxi/bus, etc. You encounter no formalities of any type.
Note, your bag must clear customs in EWR (your first point of entry into the U.S.) because once in the U.S., there are no secured international transit areas in any of the airports, and there is no immigration or customs inspection made of passengers on domestic flights. Thus, you must go through immigration whenever arriving in the U.S. Otherwise, you could simply walk out of the airport without any processing.