Ιάκοβος is correct.
Inside a house, all those words refer to the bathroom. Λουτρό and μπάνιο are geared more towards cleaning the body, while τουαλέτα and αποχωρητήριο are more synonyms to WC.
λουτρό : Special room with sanitary equipment (like washbasin and bathtub), for cleaning the body and taking a bath.
μπάνιο : Same like "λουτρό", a little more informal. Also means “taking a bath” or "swimming in the sea".
τουαλέτα : toilet, bathroom, restroom, gown (formal evening dress).
WC : Same like the English word WC. Usually public bathrooms are marked as WC.
αποχωρητήριο : WC, lavatory.