Once again, thanks to everyone for your helpful replies. I have got answers to a few questions that I am yet to answer :-) and almost all questions I asked. I am just trying to understand the system - 99% chance I will not be using any of these lines (except RER C, the one with the nicely painted interiors I hope, to Vesailles) - because I am a train fan. Unfortunately, even though I tried google translating some of the terms, they did not help me understand everything. Back to your responses ...
Sam, thanks for the explanation about your map - I have likely come across it already (I have a large version of the metro and a pocket version of it already downloaded and printed). On some of these sites, the portal gets translated to English (or an English version exists) but once I navigate a couple of pages, I am back to a page that is not translated. "Letters above that are suburban trains that terminate at various Paris train stations. Those ones generally require a separate ticket." Yes, I saw N and U lines that were in the SW quadrant. I take it that they go somewhere South west of the Ile de France region.
Lisa: I am quite familiar with only a couple of the metros around the world. 24 years ago, as a grad student, when I had a 12 hour layover in London enroute to India, I traveled the London metro with no google, no internet, no (IIRC) Rick Steves (or his forums). I have traveled the DC metro often. Last year, with my tween son, I spent a few hours in Zurich also on a layover (though most trains there are above ground). So, while I have not traveled around as much as I would want to, I am pretty comfortable with metro system. I really want to understand the transport system in Paris - don't worry, I won't get confused :-)
Jean: "The metro map will give you an idea of walking distance between lines. If you see a longer white bar across the metro stop, there's more lines intersecting, so a larger chance of needing to walk farther, i.e. Les Halles (more walking) vs. Concorde (less walking)." That is what I was assuming but I was seeking confirmation, as none of the sites I visited (including parisbytrain, transilien or RATP) appeared to have this information spelled out. Sam, you explained this as well :-)
Chris F: That was very helpful. To summarize, one can only transfer from train to train (metro to RER) with one t+ ticket. If you want to take a train and then a bus (or the other way around) to reach your destination, you need two t+ tickets (valid for the further region). Do you need to validate your ticket to exit as well? IIRC, in Washington DC and London, you need to do that. In other words, if you enter the subway with a ticket but then lost it, you cannot exit "legally" - is that right?
Appreciate your help,