I was wondering what the best way to add detail, meaning things like defining car doors, hoods, etc. By this I mean the black or darkened line around the afformentioned parts. Is it modeling, material, or something else? How? As always please give as detailed an explanation as possible.
Best is to model it,…the real trick is, if you are using subsurfs, and you need an edge that is rounded but not too round, you must put multiple edges very close together, so the subsurface does not ‘baloon’.
Use the K button in edit mode to bring up the Knife menu. Use Face loop cut, select face, left click, hold CTRL and move mouse to 80-90% left click.
Do this where you want your door edges and to sharpen up subsurf corners. Don’t be afraid to mess up cos we now have multiple mesh undos. 
Hope this helps.
Sonix.
I’ve seen some awesome car models here. Couldn’t someone with those extremly fine tuned skillz write a tut showing the tricks they use to get that super finish…like the paintjob (both the polished and the rusty ones), getting reflections on a nicely buffed hood right. The ways to model the body and so on…or have I just missed all those tuts…
Those tutorials are out there, you just have to search a little. There are many more than I listed below. Also, if your interested take a look around the WIP or Finished Works forum for ideas and techniques on what you should do and how you should do it.
Here are several that I found:
Modelling:
Recent one from ReSeT on car modelling:
http://edvberater.serverpool.org/tutorials/car_tutorial_english.htm
Paint:
A .blend with some carpaint in it that you can tweak and duplicate from Sonix I believe (correct me if wrong):
http://home.columbus.rr.com/samadam/chromaflare.blend
and another page from Sonix:
http://www.free-webspace.biz/sonix/Miner/Conceptraynewcolours.html
Another .blend (not sure who from):
http://jeeves.50free.net/blender/carpaint.blend
Also take a look at this thread:
https://blenderartists.org/forum/viewtopic.php?t=23172
The best way to add details to cars and other objects, in my opinion, is a combination of actually modelling some of it and using texture bump maps whenever possible…
If you just model “detail” you’ll end up with way too high polycount, which will dramatically increase the rendering time (which may not be too bad for stills, but in the case of an animation it could make a big difference…)…
A reasonable combination between those two, is the perfect choice…