Please keep all question here related to details of the graphics settings, and how they affect performance. Bug/crashes/etc should be directed to the support forum. Thanks.
Rather than reply to any specific thread, I'll just make a new one.
Graphics Quality - This basically controls lighting detail. On highest, every light is per-pixel, and we use fancy subsurface scattering on character faces. Lower levels gradually reduce this. Quality also directly effects which shader model is used to run the game. Highest uses shader model 3.0. High and Medium use 2.0. Low uses 1.2. Reducing this from Highest to High can have a drastic impact on frame rate. If you have a very old video card, reducing to Low may help as well.
Shadow Quality - Controls the quality of the shadow filtering. Can have a pretty big effect on frame rate. Shadows are only enabled when Graphics Quality is set to Highest.
Aftereffects - This turns on depth of field, motion blur, and glow. Big frame rate hit. Only enabled when Graphics Quality is set to Highest.
Texture Quality - Setting this to low will cause all texture resolution to be cut in half. Generally this won't effect framerate unless you have an old video card. Depending on how much system RAM and video RAM you have, setting this to low may reduce hitching.
Anti-Aliasing - Turning this on will smooth jagged edges. When graphics quality is set to Highest, MLAA anti-aliasing is used. When quality is set to High or Medium, FXAA is used. When graphics quality is set to low, no anti-aliasing is used. This usually doesn't effect frame rate too much in my experience.
Resolution - Reducing resolution can help frame rate quite a bit.
These settings are more or less the same on both PC and Mac. For the curious, both Xbox and PS3 use pretty much highest settings, except for slightly reduced shadows. iPad2 runs more or less with Graphics Quality at Medium.
Feel free to ask for further clarification. I'd also be curious if anyone has any experience with settings -> framerate that differ from what I've said here.
[Edit] - Made sticky. If one of the other regular forums mods feels this would be better merged with the general FAQ, I'm all for that too.