Estimate is quotation given to customer for services or products which you want to sell.
Purchase Order (PO) is a commercial document and official offer issued by a buyer to a seller, indicating types, quantities, and agreed prices for products or services. Acceptance of a purchase order by a seller forms a contract between the buyer and seller, so no contract exists until the purchase order is accepted. It is used to control the purchasing of products and services from external suppliers
Invoice is the bill (full or partial) which seller gives to buyer for payment against the products or services sold to buyer.