What is Music: Short Answer
Music is simply the intentional application of rhythm and melody. Because music has to be intentional, sounds such as a train running down a track, or a tree branch tapping on a window are not musical, at least not until someone uses the sound specifically as a rhythm. This means music is purpose driven and exists only as an intent.
The difference between good and bad music lies in what the intent of it is. Good music is created artfully and with significance in some sense. Bad music is created because someone felt like putting some notes together, and for no other reason. Another related, but maybe not as important quality is how we individually feel about music—our tastes. Some people like music that is extremely complex, or unusually unique. Some people only think music is good if very few people listen to it. I, personally, prefer music that is made with an intent that I can connect with as some form of entertainment, or emotional expression, or an artful representation of some abstract moment. An example of that last type of connection would be Pachabel’s Cannon in D. Cannon isn’t really about expressing an emotion, at least not profoundly, nor is it about entertaining people’s spirits. Listening to it will put you in some state of mind, though.
The purpose of music is not expression. That is something I hear a lot and I disagree with it entirely. Music can be used as a means of expression, but it doesn’t have to be. The purpose of music is to connect with others. Most contemporary, non-orchestrated music connects the artist to the audience, but other forms of music connect the audience with each other, leaving out the artist from the connection. Pomp and Circumstance is a song that is used in this way very often, as a way of celebrating achievement and comradery.