Classical conditioning we already discussed. By pairing a neutral stimulus with something that has meaning, we give the neutral stimulus meaning. We can condition a benign beep to predict a shock or a click to predict a treat. We are aversively or appetitively conditioning. Just simple pairing.
Desensitizing is the opposite, reducing the meaning. So if beep=shock or click=treat, you can progressively repeat the stimulus without it's pairing until the meaning fades. Clicking repeatedly without pairing it with food will progressively remove it's value down to nothing. Same with the beep.
Counter conditioning is when you change something's meaning. You take a stimulus that was aversive and pair it with an appetitive until it takes on that meaning. So pairing the beep with food and no shock. Pairing the click with a shock and no food would counter condition the click to become aversive (we call this "poisoning").
This happens all the time throughout our life without us noticing, but we can also use this in intentional training to help change the way our learner feels about something.