公共 (gōnggòng) is an adjective meaning 'public, common, communal' — it describes things that belong to or serve the general public, like 公共汽车 (bus) or 公共安全 (public safety). It differs from 共同 (gòngtóng), which means 'shared/joint' and typically pairs with abstract nouns (e.g., 共同目标, shared goal). It also contrasts with 公开 (gōngkāi), which emphasizes openness or making something public.