Discipline is a SYSTEM. Every parent, from the authoritarian to the overly permissive, has a discipline system in place. Some systems are more effective than others, sometimes the effectiveness is dependent more upon the child than the system itself. As a system, discipline is neither inherently "bad" or "good." It is effectively neutral. The resulting behaviors [see a future blog post on BEHAVIOR/BEHAVE] can be interpreted with these affective labels, but the system itself should be seen as either effective--does it accomplish the goals it was designed for--or ineffective--does it fall short of expectations. As a system, many parents and teachers and bosses (and anyone else in a position of authority over another person or animal) choose to use both punishment and reinforcement to direct (or guide) behavior. Let us clarify these terms now. But first, the word "consequence" needs to be defined for you so that you understand the rest of this post.
Consequence is anything that follows an action (i.e. behavior). Consequences never come before a child acts. If you yell at your child before he/she hits their sibling, then you are employing an antecedent. These are used for classical conditioning and are mostly effective for reflexes, but not for conscious or voluntary behaviors. So, I will focus on consequences for the duration of the explanation of "discipline" as a system.
Punishment: this is a consequence that results in the reduction or cessation of a behavior. If yelling at a child when they spill something leads to that child never spilling again or reducing the frequency of spilling, then you have instituted a punishment. If you remove a toy from a child when they argue with their sibling and they stop arguing, then you have used punishment.
Reinforcement: this is a consequence that results in the repetition or increased frequency of a behavior. If thanking a child for cleaning up their mess results in them cleaning up more messes, then you have used a reinforcement. If you hug a child when they are sad and they come to you more often for hugs when they are sad, then you have reinforced the behavior (not sadness, but seeking your comfort when sad).
Important Note: These consequences--punishment and reinforcement--are defined by the individual receiving the consequence, NOT by the person doling them out. For example, if you yell at your child for spilling their drink and you find them spilling liquids much more frequently, then you have actually reinforced the spilling behavior. There is something about your yelling that the child desires, hence they will continue to behave in such a way that will allow them to receive more yelling. If you give your child candy for cleaning their room and they stop cleaning their room, then you have actually punished them. Perhaps you should ask them if they like the candy.
Now, as a teacher I don't like to leave things incomplete if I can help it. However, the complete ideas of punishment and reinforcement as discovered by B.F. Skinner and other behaviorists is a little more complex. Included in this area of psychology are positive punishment (giving something to the person that they do not like so that they won't repeat the behavior), negative punishment (taking something away from the person that they like so that they won't repeat the behavior), positive reinforcement (giving something to the person that they do like so that they will repeat the behavior), and negative reinforcement (removing something from the person that they do not like so that they continue the behavior). There are also ideas of schedules of reinforcement (fixed interval, variable interval, fixed ratio, variable ratio, continuous) as well as types of reinforcers (primary, secondary, and social). Anyone who has ever taken a well-taught introductory general psychology class or a class on behaviorism or learning and conditioning will know more of these details. If you haven't, then pick up a psychology textbook and peruse the chapter on learning.
When you "discipline" your child or your employee or your student or anyone else, you are shaping behavior, not merely punishing them. Incidentally, decades of research has found that reinforcement of wanted behaviors is much more effective in shaping behavior than punishment of undesirable behaviors. And, if you do choose punishment as part of your disciplinary system, then you really need to 1) explain why that behavior is being punished so that it can be avoided in the future and 2) let the individual know an alternative, acceptable behavior that they can pursue to replace the undesirable one. Otherwise, a person ends up with a gigantic list of "don't" and has no idea what the "do" might be. Another key component to any discipline system is consistency. Without consistency people will not know what you expect from them and life will become one big dice game that is completely unpredictable.