People ask this a lot -- and it is important to distinguish between the great work that the QGIS community does to make such a powerful application. The Boundless Desktop team is comprised of active members of the QGIS community, who contribute to the development and maintenance of the QGIS project. Boundless values and invests in QGIS, which is the core of Boundless Desktop and the starting point from which the company provides desktop support, training, add-ons, and services.
Boundless Desktop is a supported application bundle of proven open software including: QGIS, PgAdmin, Qt Designer & Python. It’s distributed through a single installer for Windows or Mac. This installer includes many common or dependent libraries for QGIS such as GDAL/OGR and others also built into the installer.
In addition to the QGIS Official Plugin Repo, it links to the Boundless Plugin Repository. Among those plugins is Boundless Connect, which opens when you launch our distribution of QGIS for the first time. Connect is the online hub for Boundless products, tools, support and documentation, and offers an ability to connect with a global community of fellow geography professionals.
Finally, Boundless offers commercial support and maintenance for our Desktop installs, which go through rigorous testing prior to release. Support also includes access to tutorial datasets, workshops and how-to videos, and access to learning resources.
Disclosure: I am the Boundless Desktop Product Manager