MGED is currently the primary geometry editor in BRL-CAD. By default, MGED will provide a graphical user interface for modeling, editing, render, and managing geometry models. MGED can also be run in a command-line "classic" mode using the "-c" command-line option, which can also be used for scripting interactions.
MGED does not provide a discoverable graphical user interface. Going through the available tutorials and documentation is required to be proficient. MGED is expert-friendly with minimal documentation and feedback inside the application itself.
MGED maintains immediate storage persistence, i.e. automatic and immediate saves. There is no need to save your file(s) as all modifications are immediately saved as soon as they are applied.
MGED (and most of the BRL-CAD tools) perform constant validity checking and will abort early at the first sign of error or corruption detection in order to minimize or prevent data loss at all costs.
MGED is intentionally a highly modal editor (similar to vi) in that there are various editing modes and states that you can go to/from while editing geometry.
MGED will not automatically display the contents of a geometry database when it is opened/loaded. Unlike other CAD systems, a single BRL-CAD geometry database can contain an arbitrary number of complete geometry models. Run the MGED tops command or use the Tools->Geometry_Browser to see the top-level objects in an opened .g geometry database.
There is extensive documentation and tutorials available for MGED under Documentation.