The words are synonyms, having very similar meanings, and for the most part are interchangeable, though "main" is more often used in everyday speech.
I enjoy the old 1828 Webster's Dictionary. His definitions for both words are:
Main: Principal; chief; that which has most power in producing an effect, or which is mostly regarded in prospect; as the main branch or tributary stream of a river; the main timbers of an edifice; a main design; a main object.
Principal: Chief; highest in rank, character or respectability; as the principal officers of a government; the principal men of a city, town, or state.
Most important or considerable; as the principal topics of debate; the principal arguments in a case; the principal points of law; the principal beams of a building; the principal productions of a country.