Unlike the G-Cloud framework which is for "off-the-shelf" cloud products, DOS is designed for bespoke, outcome-based work. It allows government bodies to find a whole team to deliver a digital project (a "digital outcome"), or to procure individual specialists to fill specific roles in an existing team (a "digital specialist"). It also covers user research services, including the provision of studios and participants. The framework's design, which emphasizes an agile approach and a focus on user needs, makes it easier for organisations to build and deliver digital services that meet the Government's Digital Service Standard.
You can find more detailed information on the GOV.UK website.