Reservation in international law means state's acceptance, approval of a treaty by ratifying , approving, implimenting it.It makes state party of the treaty excluding the clauss it doesn't support, A reservation is made everytime the treaty affects the state. It has well laid objectives and procedure for implimentation .

A treaty has singned applicants and parties who legislate it whereas a Declaration is UN declaring something or other without any signatories and party involvements.If state signs treaty it is obligatory even if government changes but declaration does not involve any obligation on state'e part.