What is difference between the terms budget and estimate?

Budget is a formal and approve document. It has two parts: How much funds are required and how these would be met. It is the same as cash inflow and outflow. Estimates reflect only expenditure side and it could be an informal way. Supposing a dam is to constructed. First will come the estimate say $5 billions. Even if this figure is accepted as realistic, the question would arise where from the money would come. On identifying the genuine sources, a budget would be prepared and approved. In short, while both show future costs, estimate point out towards expenditure while the budget would also give details on financial plan or how that expenditure would be met.