These are brief overview summaries of "key aspects of the Department of Defense and how Congress exercises authority over it" developed by the Congressional Research Service (CRS), the research arm of the Library of Congress. They are specifically aimed at helping new members of Congress but they also provide excellent overviews for the average citizen as well. More CRS reports can be found here.
The tabs are organized to match the source "Defense Primer: A Guide for New Members" [R44757] which is a consolidated list of the various reports. As new additions come out, they will be added to the appropriate section.
Image source: U.S. Army