tl;dr : Go find it out yourself kiddies.
Well this one is for obvious reason and people keep on saying they have lag and shit and here is info for the graphics and what they do.
Resolution : Makes the Textures less pixeled recommended to turn it down for less lag.
Brightness : Just makes the game lighter or darker recommended to not touch or you may mess it up.
Specular Map : Dont turn it on if you got a shit card or low ram or cpu recommended to keep it off for less lag.
Shadows : Well its an obvious setting recommended to keep it off for fair FPS.
Number of Dynamic Lights : Its like Lighting except its lights so I keep it off for fair FPS.
Model Detail : It makes the guns,people,cars and other things lower the graphics or makes it higher.
Glow : Obvious setting recommended to turn off for FPS.
DOF (Depth of Field) : It changes how far you can see recommended to turn off for FPS.
Sync every frame : If you got a shit card turn this off cause it will make you lag and it makes... I dont know.
Soften Smoke Edges : Obvious setting also turn it off for FPS.
Ragdoll : Obvious setting again it doesnt affect FPS and its fun to look at.
Bullet Impacts : This controls the blood also so turn off for FPS.
Water Detail : It is the graphics of the water turn it down for FPS.
# of corpses : Turn it to lowest for fps.
Dual Video Cards : First time I thought this will fix some lag but I was wrong this is for Dual core cards.
Anti-Aliasing : Turn it off for FPS and I still dont know what it does.
Texture Settings
Texture Filtering : Changes the graphics a little keep it to Bilinear for FPS.
Texture Quality : Changes the Graphics.
Texture Resolution : Main graphics part of the game keep it to low for good FPS.
Normal Map Resolution : Changes some pixels keep it low for FPS.
Specular Map Resolution : This will get the Specular map more detailed.
Hope you liked this info for the graphics part. Tell me if I missed anything.