They are different, but linked. It's a little hard to explain to anyone who has not used manual equipment. Don't know if you have or not. Nowdays, with a full auto AF body, if you change anything like ISO or aperture, shutterspeed, etc., the exposure still comes out OK. So there must be a way to leave the camera in auto mode, but still have human intervention. First is exposure compensation. This will make the overall image, whether flash used or not, darker or lighter than the meter dictates. Second is flash compensation. Use this to increase or decrease the output of the flash only. It does not affect the existing light in the scene. The two can be used together, but remember that exposure compensation rules. If you increase the flash by +1, and decrease the exposure by -1, you have made the "background" darker, but the flash is effectively unchanged. There are many situations where the meter is fooled, and either exposure or flash compensation should be adjusted, but there is little justification for routinely leaving one or the other arbitrarily set. This is all very general information that applies to any brand or model. If you want to see them in action, shoot a roll of slides, keeping notes for each frame as to what you have changed.
"Less is not more. Enough is enough. Less is less." David Vestal
"I am always doing that which I cannot do, in order that I may learn how to do it." Pablo Picasso