礼貌 (lǐ mào) is the general word for politeness, courtesy, and manners—it can be a noun ('politeness is important') or an adjective ('a polite student'). 客气 (kè qi) also means 'polite,' but specifically refers to courteous formality between people, often with a sense of standing on ceremony or being overly modest; it's used in set phrases like 别客气 ('don't be polite / make yourself at home').