A contractual lexicon.
The contract flow can be seen as such:
NDA (Non-Disclosure Agreement)
Before signing a contract, a NDA (Non-Disclosure-Agreement) is needed in most cases in order to save both sides interests.
MSA (Master Service Agreement)
Defines how the two companies will do business together.
SOW (Statement of Work)
Describes the work to be performed for the client. Note that the MSA defines the “how” and the SOW defines the “what” of the work.
CO (Change Order)
Change Orders document any changes in scope, budget, timeline, assumptions or any other deviation from the current SOW.
PR/PO (Purchase Request/Purchase Order)
Usually for bigger companies - in this case in most cases no MSA or SOW needed
You can read more here.