违反 (wéifǎn) means 'to violate, to break' a law, rule, or agreement — it's a general, formal verb for going against something that has been formally established. 违背 (wéibèi) is similar but broader, also covering abstract things like wishes or principles; 违规 (wéiguī) is narrower, specifically 'to break regulations' (规). 违法 (wéifǎ) is a fixed term meaning 'to break the law' and can also be an adjective ('illegal').