The answer about the difference is fairly simple, and no one has really
mentioned them.
What was mentioned was that AutoCAD is the main basic program. It has all
the text, basic entity and drafting abilities, including loads of 3D
capability using solid modeling. It even has the rendering.
ACA is a whole slew of additional functions specifically for architects and
the building construction industry. This includes highly customizable
walls, doors, windows, roofs, slabs, stairs, structural layouts, plumbing
fixtures, basic lighting, electrical and HVAC layouts and the ability to tag
and schedule all those things. There's also the "intelligence" of these
objects - doors and windows only insert on walls, and if you move a
door/window, it only moves along a wall cutting all parts appropriately.
Since most of the main building parts are also in 3D, you have the ability
to cut sections, pull elevations, and update the those basic views after
making changes to the model. Then there is the whole wall sections, plan
details and component detailing thing with the ability to reference the
information back to the main plans for construction documents. There's also
the layering standards.
All these things could be drawn in plain old Acad too, but you'd be drafting
everything from scratch - including cutting and trimming your wall around
your doors/windows. Make a simple change and you have re-cut your openings
in the new location and clean up the old one - especially if the change
involves moving something to another wall. All references and tags have to
be input and kept track of manually.
ACA can't do everything - there are still things that have to be "drafted"
sometimes, but the biggest part of the work can be done for you. Luckily,
since ACA in built on top of Acad, all the basic Acad drafting tools are
there too.
Vanilla AutoCAD began adding some drafting shapes for steel and drafting
symbols (like a section cut symbol), but all of those were extremely
limited. Steel shapes were missing about half of them, for example.
Mostly, I think, it was to show off the new dynamic block capability that
started in '06.