What are the common HTTP methods and their purposes?

GET retrieves data (should be idempotent, no side effects). POST submits data to create resources (not idempotent). PUT updates/replaces entire resource (idempotent). PATCH partially updates resource. DELETE removes resource (idempotent). HEAD retrieves headers only (like GET without body). OPTIONS describes communication options. Common status codes: 200 OK, 201 Created, 400 Bad Request, 401 Unauthorized, 404 Not Found, 500 Server Error.