purpose: what you want to achieve when you do something; the reason you do or plan something, and the thing you want to achieve when you do it: The games have an educational purpose. aim: something you hope to achieve by doing something: The main aim of the plan was to provide employment for local people. goal: something important that you hope to achieve in the future, even though it may take a long time: The country can still achieve its goal of reducing poverty by a third. target: the exact result that a person or organization intends to achieve by doing something, often the amount of money they want to get; a particular amount or total that you want to achieve: The company is on track to meet its target of increasing profits by 10%.

In use they are interchangeable.