The MSA (Master Services Agreement) is the umbrella legal agreement between you and GSL. You sign it once and it defines the general terms: payment, confidentiality, intellectual property, client responsibilities, liability, termination and dispute resolution.
The SOW (Statement of Work) is attached to the MSA and is specific to one project. It defines the selected service or package, exact deliverables, price and currency, payment schedule, start and completion dates, number of revisions, items excluded from the price, client materials and approvals required, the acceptance process and the additional-work rate.
Example: "Five-page business website for $1,200 USD, including two revision rounds."