You already know what a Indian mainstream movie is for e.g any movie where there is a actor/actress u or everyone recognize for e.g Mr. Bachhan, Salman, Aamir, SRK, Deepika, Katrina etc.
In a B-Grade Cinema you will only recognize one actor who also acts on mainstream movies for e.g Shakti Kapoor, Mithun Chakrabarty, Dharmendra etc. (Just search in Youtube for Gunda and you will know). These movies may have sex as their theme but may or may not have nudity. Also, they may not always have sex as their theme. These are low budget movies, have terrible/rarely good scripts, bad acting, terrible camerawork. Just google Kanti Shah and Sapna/B-grade Sapna to know more.But in C-Grade movies you will recognize no one. They are a sub set of B-Grade movies minus recognizable actors. These movies always sex as their theme. Always have nudity (depends on the movie theatre for e.g the ones where rickshaw wallas and trolly wallas go show nudity but in big theatres having repute they dont show nudity). These are super low budget movies, have terriblescripts, bad acting, terrible camerawork. Just google Zakhmi Naagin, Kunwara Paying Guest, to know more.