difference between a contract and a purchase order
Consider these examples. When accepted by a vendor every PO becomes a contract, but not every contract is a PO. Some (but not all!) people view POs as a kind of one-time contract. Purchase Orders may cross-reference the terms of a negotiated contract. It's a common misconception that the level of detail in a PO is less than that of a contract.
Legality: A contract is a legal document and a PO is a commercial document. To elaborate, a PO is an offer from the buyer to the supplier. If accepted by the supplier (within a reasonable timeframe) the PO becomes a contract. Logically, this PO will have zero value if it's not accepted by the supplier.