What does post processing actually do?

Can anyone tell me what the various levels of post-processing does?

Using this wiki I was able to disable DOF and possibly Chromatic aberration which really helped with the horrendous blurry main menu.

But, I wanted to know if there are any good post processing effects that I would benefit from by having it on Ultra setting.

If anyone is curious, putting the following under [SystemSettings] in you Engine.ini greatly improves the image in the menu.

r.SceneColorFringeQuality=0

r.DepthOfFieldQuality=0