What's the difference between a purchase order, a purchase agreement, and a contract agreement?

Oracle Purchasing provides the following purchase order types: purchase order, blanket purchase agreement, and contract purchase agreement.

Note that your organization can change the names of these documents to suit your business needs, but in their basic form the following describes their function:

• Purchase Order You generally create a purchase order for a one-time purchase of various items. You create a purchase order when you know the details of the goods or services that you require, estimated costs, quantities, delivery schedules, and accounting distributions.
• Blanket Purchase Agreement You create blanket purchase agreements when you know the details of the goods or services you plan to buy from a specific supplier in a period, but you do not yet know the detail of your delivery schedules. You can use blanket purchase agreements to specify negotiated prices for your items before actually purchasing them. Prices for items can be negotiated on a blanket purchase agreement by a procurement business unit which can then be made available to multiple requisitioning business units.
• Contract Purchase Agreement You create a contract purchase agreement with your supplier to agree on specific terms and conditions without indicating the goods and services that you will be purchasing. You can later issue purchase orders referencing your contracts using terms negotiated on a contract purchase agreement by a procurement business unit which can then be made available to multiple requisitioning business units.