Power limit: adjusts the temperature point that the GPU core will throttle. Throttling is where the core frequency drops on its own to lower the temperature.
Core frequency: the actual frequency the GPU core runs at. Adjust by 10 MHz at a time testing each time till errors appear then lower by 1 MHz till errors stop.
Shader frequency: this is an internal computational component in the GPU, effect gaming ability also. Adjust the same way as the Core frequency. Best to adjust this separately, Asyncronous. There is a setting for this in the settings for afterburner.
Memory frequency: this is like ram frequency. Adjust the same way as the core and shader settings testing each time. testing is very important and can not be skimped on.
Use Prime95 and Intel Burn Test to test CPU stability.
Prime95: run the Blend test for 2 hours to start, then once you have the desired OC then test for 12 hours.
Intel Burn Test: run this program 10 run on maximum stress till you get the OC you want then test for 20 runs at maximum stress for final stability test.
Why 2 tests? because there are times an error will slip through on one program but the other will catch it. This is why the redundancy. OCing will take a few weeks of testing and tweaking to finish but well worth it.