The 'traditional' way of CAD for architecture is to draw a bunch of lines and circles, much like you'd draw a plan by hand, but on a computer. This is very error prone and labor intensive. Your client asks you remove a window, you need to remove it in several views etc.
The holy grail of architectural CAD is in a part of what is loosely called 'BIM', Building Information Modeling. You make a 3d model of a building and generate separate views on that, like cross sections and plans, but also use that same model to do e.g. earthquake simulations or energy efficiency analysis or cost estimating. Autodesk has a product called Revit that purports to do this, but it has a long way to go to really fulfill the 'BIM dream'.
This is a subject near and dear to my heart - working with architects and contractors who run around with A0 format paper copies of plans, who do cost estimates by measuring, with a rule (!) each room on the paper plan and calculate areas with a calculator on a piece of paper, it makes me die a little bit inside each time I get a quote and I see the areas or dimensions or whatever different for each fricking ocntractor, instead of using software that would do pretty much everything that they just spend a day on in a few seconds.
Most 3D modelers are solid modelers (i.e. I construct a solid cube or sphere). When I aggregate many of these solids together (and possibly do some boolean operations to them), I have a design.
Now I would like my 2D cuts. By this, I mean I would like to look at a 2D cross-sectional drawing of my design. This may be at the XY plane, YZ plane, or some other arbitrary plane.
Most 3D modelers can do this, but when you make the 2D cut, you're often exporting the cut to a new file. If you change anything in the design, the cut does not update, and you must "re-cut". This is what I mean by real-time.
Good 3D modelers don't do this well, and poor 3D modelers can do this exceptionally (I'm looking at you Revit). Some can do both, but they're incredibly complicated and cost tens of thousands (Digital Project)
But for architectural design, you want to work at a higher level than solids. You want to draw a 'wall' and then parameterize it - 8cm of outside brick, 2 cm of air, 5 cm of insulation, 14 cm of brick, 2 cm of plaster. Plus this needs to have the right hatch in 2d views etc.
Have you tried Chief Architect? It works rather well in this respect, it's really made for ease of use. I don't know how well it would scale for large projects though, I've only toyed with it for small residential design. Plus the estimating and electrical/HVAC tools are poor to non-existent.
I'm taking the view of an avant-garde designer in a firm that has a separate team of drafters to implement designs and construction documents. My focus is on the conceptual design phase, so I simply have different needs.
There are a number of programs, but most designers I know prefer Rhino (rhino3d.com) and the Grasshopper (grasshopper3d.com) plugin. Rhino is now adding Python (and OSX) support, but they still have a ways to go.
Never mind my other reply, I hadn't seen your response above. I had a quick look at Rhino, I hadn't heard about it before. I'm a bit surprised that you work with the same tools as e.g. an industrial designer, although when I think of it I can understand, with the lack of expressiveness in the specialized packages and all.
"plans" means 2D overhead views of a 3D model. "sections" means 2D side views of a 3D model, like a cross-section. I don't know what "real-time" means here, but I'd bet that it means you can grab onto the plane that defines the cross-sectional cut, move it in the normal direction, and have the model update smoothly. (The non-real-time alternative is that the location of the plane is specified by a value in some dialog box.)