Diff b/w PO , Schduling agreement and Contract

Hi Sap Gurus,

Please tell me the diff b/w PO and Scheduling agreements and Contract.Why i can't use PO only instead of other two. Suppose i am creating Contract for 1 year but i can create PO with del date for 1 year and monitor the delivery dates

Please any body explain in detail with examples. Thanks in advance

Regards,

Ravi