stall: when the butterfly closes, and the pressure has nowhere to go (be it a poor bov or no bov at all). the only way the pressure can vent is back through the turbo, even though the turbo is still spinning. this is what causes the chopping noise. alot of people call this "wastegate chatter" which is also wrong. surge: this is when the compressor produces more boost than the inducer/engine can cope with. the engine can only "breathe"so much air, and if the turbo is flowing more CFM than the engine can process, then you have a problem. thats why you get surge protectors on the bigger turbos (those holes that are drilled into the intakes) its feels sort of like a horrible missfire when it happens and the turbo makes a horrible stammering noise, totally different from the chips you get when it stalls. thats about as basic as i can explain it. 99% of people dont know what surge is, as they never see or experience it, as it only happens way up the tuning ladder. that, coupled with alot of ill-informed people has sort of marred the definition and everyone calls every noise a turbo makes "surge"