please check the following link for the definition and the documentation: <a href="http://help.sap.com/saphelp_47x200/helpdata/en/75/ee0e9755c811d189900000e8322d00/frameset.htm">http://help.sap.com</a>
In the left tree you will details about the contract and the scheduling agreement.
Hope this helps.
Outline Purchase Agreement are subdivided into:
Contracts (Qty / Value) </b> (Centrally agreed / Distributed ) against which release orders (releases) can be issued for agreed materials or services as and when required during a certain overall time-frame.
<b>Scheduling agreements</b> are created referencing a centrally agreed contract, where in materials are procured on predetermined dates within a certain time period.
Also, check out pgs 58 onwards in this PDF :
Please check this link.
Hope this will help.