The term, precisely
A master services agreement is an umbrella contract establishing the general terms that govern a commercial relationship: liability caps, intellectual property ownership, confidentiality, termination rights, payment terms and dispute resolution. Individual engagements are then defined in statements of work that sit beneath the MSA and inherit its terms automatically.
Why the framework outlasts the deal that justified it
MSAs are usually negotiated around a first engagement that is deliberately small, and the legal review is sized to match. Two years later the tenth statement of work is twenty times larger, and it is still governed by a liability cap, an IP clause and a termination notice period that were agreed under time pressure for a pilot. Renegotiating mid-relationship is possible in theory; in practice the supplier now holds the operational dependency, and knows it.