Reinforcement is any event that strengthens or increases the likelihood of a behavior occurring again. For example, if you receive praise from your teacher for completing your homework, it may reinforce the behavior and increase its frequency.
Positive punishment involves adding an aversive stimulus after a behavior to decrease its frequency. An example would be getting detention for talking in class, which may reduce future occurrences of talking during class.
Shaping is a process where behaviors are gradually modified by reinforcing successive approximations towards a desired goal or target behavior. For instance, if you're learning to ride a bike, someone might initially reward you for just sitting on the bike before moving onto pedaling and balancing.