You pat something when you touch it with the palm of your hand. People usually pat dogs and cats on the head, or give each other a pat on the back for a job well done.
A very similar word to pat is stroke, where you run your hand along something slowly as you are patting it.
A peck is a quick movement made by birds when they eat, hitting their beaks against something. Birds also peck to do damage, for example woodpeckers make holes by pecking.
A movement similar to a bird's peck is also called a peck, for example "she gave him a quick peck on the cheek".
You pluck something by pulling it off at the growing point. People commonly pluck out hairs from their eyebrows, pluck fruit from a tree or vine, or most commonly pluck the feathers from a bird like a chicken to prepare it for cooking. The chicken in the picture has been plucked everywhere except its wings.
Plucking can also describe a short, sharp way of playing an instrument like a guitar, or even be used as another word for courage, as in "he has a lot of pluck".