This is a really tough answer to articulate well, and one of the smarter folks here will answer it much better than I can.
When looking at a three prong 120v wall outlet in the US the small prong is hot, the large one is neutral, and the hole is earth ground. Earth ground and neutral connect at the circuit breaker box. Your conduit should be connected to earth ground for safety. That way if your hot comes loose and touches the conduit or your hot comes loose inside of your metal enclosure it will blow the breaker instead of electrifying the conduit or metal enclosure. The neutral is also your 0v reference. If you measure voltage with your meter between ground and neutral you'll measure 0v. If you measure between ground/neutral and hot you'll measure the AC voltage coming in to the house.
Inside your stompbox "ground" is just a zero reference point. Your stompbox enclosure ends up connected to the negative terminal of the battery when you use non-isolated 1/4" jacks.
EDIT: My explanation sucks. I'm going to put together some stuff written better and post it later today.
-ve and ground MIGHT be used interchangeably, but -ve makes me think there may is a bipolar supply. In that case you'd have a 0v (ground) +ve and -ve. This can be obtained from putting two batteries or caps in series then using the point where they join as 0v (ground) and the negative side for -ve and the positive side for +ve.