What is Reinforcement?
Reinforcement, or a reward, is anything your dog will work to get. If a behavior is happening more often, then it is being rewarded in some way. Many behaviors are self-rewarding and they will happen more often over time, since the dog simply enjoys doing that behavior or enjoys what it gets from doing that behavior. An example: Many dogs enjoy barking and over time will bark more often. Another example: dogs generally like food and free food even more, so counter-surfing is self-rewarding. Dogs love to smell good stuff, so even the chance to smell what is on the counter will reward counter-surfing behavior.
Reinforcement is done in two ways - adding a good thing or taking away a bad thing.
If you give your dog a treat when she walks with you on a loose leash that will happen more often. Likewise if you only move forward when the leash is loose, that behavior will happen more often. Both are examples of adding a good thing to reward behaviors. Reward based trainers use this kind of reinforcement most often.
If the dog's behavior makes something "bad" go away (taking away a bad thing) then that behavior tends to happen more often. Reward based trainers are very careful about using this kind of reinforcement, because you have to add a "bad" thing before you can take it away. So reward based trainers only use this type of reward in very special cases, such as when the bad thing is already there.
For example: One special case would be teaching your dog to not fear something, since the scary thing must be present in order to work on becoming less afraid of it. To reward good behavior in the presence of the scary thing, either the dog can move away from it or the scary thing can move away from the dog (plus give treats at the same time). Both ways take away stress and scared feelings, so the calm not-afraid behavior is rewarded and will happen more often.
Please be very careful if you try this by yourself. Working with dog fears and anxiety can be tricky. Any work dealing with fear or anxiety must be done below the dog's fear threshold. So, if your efforts seem to be making the matter worse or if the dog can't eat the treats when the scary thing is there, please find a skillful trainer that uses only reward based training to help you.
Gimme here: When I was afraid of horses, my person never made me go close to them. Forcing me to get close to something I was afraid of would only have made me more scared. On top of that, I would have worried whether my person could be trusted; I need to know my person will keep me safe.
Instead, we watched the horses from far away where I wasn't afraid. My person also gave me lots of peanut butter to lick up while I was watching the horses. Peanut butter is the BEST! I sure love that stuff. Every time I saw horses I got to eat peanut butter, so I very quickly learned when horses are in sight, peanut butter is on my lips. It took many months, but now I like horses a lot better. Just recently I sniffed a horse's nose and let him sniff me. I'm getting braver all the time because my person doesn't force me to do something I am not ready for.
The one who is receiving the reward (in this case the dog) is the one who "decides" what is rewarding and what is not and how much so. What one dog loves another may not like at all. Some dogs like food the most and others are more interested in toys. Many things that are in the dog's world can be rewards. If the dog wants it enough to work for it and if you can control access to it, then it is usable as a reward.
My dog Michael was stressed by strange places and wasn't so sure he wanted to do agility. Every time I tried to get him to do something, he ran off to the edge of the field to pee on bushes. I knew this was a sign of how worried he was and that he wasn't being disobedient, he was trying to relieve his stress. I also knew that I couldn't fight him on it if I wanted him to get over it and to one day enjoy agility. So I put him back on leash and got him to jump one very low jump and then I ran with him to the bushes so he could sniff and pee. After he'd finished, we went and did two jumps and then ran to the bushes. We repeated this sequence, sometimes harder and sometimes easier. It only took a couple of sessions before he realized that this place was okay. By respecting his needs and using what he wanted (and needed) to do as a reward, he came to love agility. Learning agility made him more confident and before long he was going to all kinds of new places without any concerns at all. Michael was halfway to his Masters Agility Championship before we had to retire him because he was too old to jump any more.
I heard of a woman who trained her terrier in agility by using access to nearby trees as rewards. Her dog didn't want to pee on the trees, he wanted to run around them looking for "little buddies" (squirrels) in the branches. Instead of fighting that huge distraction, she used it to great advantage. A dog who can win access to something that distracts them, won't be distracted by it because they know they can have it if they work for it. Dogs that learn how the training game works are willing to work for stuff.
You and your dog will have different ideas about what is reinforcing. In this situation, the dog's ideas win. If you observe your dog closely you will find out what they want. How many things other than food and toys does your dog want? Make a list and then be creative and find a way to use those wants to your advantage.
Gimme here: I learned on the first day I had my new person that she is very likely to give me what I want if I sit pretty first. So if I want to go out the door, I sit. If I want to get out of my crate, I sit. If I want out of the car, I sit - then she puts my leash on and off we go. If I want her to play tug with me, I bring a toy and sit. If I want her to get off the computer and pay attention to me, I sit. Sometimes you have to nudge them with your nose so they see you are sitting pretty. Still, most people simply cannot resist a sitting puppy. It may be most powerful thing you can teach your person. I'm just saying.
And remember, things that are rewarding to a dog on one day or at one time, may be aversive or neutral at another time. A dog that has just eaten may not work for treats. A dog that is really tired, may not work for the chance to play tug with you. But both of them may work for a belly rub.
Rewards are not as effective when a dog is under stress. In fact, if a dog is really stressed or fearful, learning cannot happen. Still you are likely to get more from a stressed dog with rewards than with punishment. Punishment only increases the anxiety making it unlikely that she can perform or learn.