MAS is the official name behind the term everyone actually uses in conversation, the GSA Schedule. GSA consolidated what used to be dozens of separate schedules into this single MAS contract vehicle, organized by SIN, so buyers across the government can find and order from approved vendors without running their own competition. If you see MAS in a solicitation or a GSA document and GSA Schedule in casual conversation, they are the same program.

In a sentence

"This is a competitive procurement conducted among MAS contract holders under Federal Supply Schedule ordering procedures." That is the same eBuy-style competition you would recognize if it said GSA Schedule instead of MAS, just written in the more formal language GSA itself uses.