Activity - how much the dog likes to be moving, how much the dog naturally moves around. Even when nothing is going on, these dogs will be moving. They might patrol the yard or the house or just like to move. Think of a human who is always tapping a pencil or foot or pacing.
Energy - how long the dog can go, how much stamina and oomph to their actions. So, a very energetic dog might be completely calm in the house or in the kennel, but when something is going on, they will go and go and go. This type of dog needs a regular "burn" of energy to stay sane, but they don't necessarily pace or fiddle or patrol or move around just to be moving.
Drive (high drive/low drive): A dog that needs to do something in response to an outside influence. A high activity and high drive dog might discover that he can play hockey with his food bowl or water bucket and will toss it into the air and bat it around and start a game going that meets his need to be active and to use drive.
A high activity dog with low drive is just sort of pointlessly in motion. A high drive dog is more likely to find some way to channel that activity--so they will be active with a purpose or target for their activity--this might be the dog that decides to dig up the sprinkler system in the yard. They make up games to serve as outlets for their need to be active and their need to be express their drive.
A dog with lots of energy and lots of drive but maybe not so much activity is not as common. I have a girl like this--if she's loose in the house, she watches the cats. She's incredibly calm and quiet and intense. But if she sees a cat out of place, she immediately goes into motion, almost explosively. I know from the training and playing with her how this applies in obedience and herding. She is intensely focused and has the energy to stay "on" for long periods of time, she doesn't have to take action just because she is in drive--but she can be explosive when she does take action.
A low drive, low activity but moderate+ energy dog might be a total couch potato in the house. He may be hard to motivate for training but you want to go for a 10 mile hike--that dog will be right with you
There is, obviously, some correlation between high drive and high energy and high activity--you're not going to have many high activity and high drive dogs who are low energy--so possibly energy is a result of drive + health + conditioning + something innate. And most active dogs are going to be pretty high energy, but you can have an active dog who just is sort of a flash of activity without any staying power--not common in GSDs, I think.
I have a Czech female with Grim, Cordon, & Tom up close and she has pretty high prey.
I understand that Grim brings a lot of prey. Cordon and Tom are going to bring some prey, but more seriousness.
When you say "reactive" do you mean "sharp" and quick to growl at something that appears threatening. Or do you mean reactive as in handler sensitive (quick to respond to handlers needs and commands, flashy obedience etc.)
I think of reactivity as how quick a dog triggers or responds to something. Thinking of an actual trigger on a gun--how much pressure does it take to make that gun go off. If only a little bit of input from your finger, that is very low threshold and very reactive. If it takes a lot of finger pressure to pull the trigger, that is very high threshold, very low reactivity. Reactivity is widespread and not just in one area of life (not just prey or defense) -- so, how fast does your puppy stimulate and decide to chase your pants cuff, a rag on a string, a grasshopper in the yard? How fast does your dog trigger to a cat beginning to walk through the room? What does it take for your dog in the car to begin to bark at that passerby?