Art is really a manifestation of thoughts, ideas, and creativity. It is essentially an output for any idea or message that the artist wants to get out to his/her audience. Art can also just be a little sketch somebody made for fun, or it can hold a much more powerful meaning. Art is really just a story, only told in pictures instead of words. Each and every piece has a story and meaning behind it, even if it is just a little sketch on the margins of a piece of paper.
Artists create art when they want to spread a message or idea. That even applies to the minor doodles on the margins of a paper. Every single piece of art has a meaning behind it, and that meaning is what the artist wants to get across. A few examples of messages artists want to get across can be supporting a movement, showing the reality of an issue, or just capturing the beauty of an object or scenery. Artists can really create powerful pieces based on the message they are trying to show.
Artistic style is really something that depends on the artist. It's the way they create their art, and how they want to get their idea or message across. The specific style of an artist can also reveal a little about who they are as a person. Really, artistic style is almost the manifestation of the artist's personality, and who they are as a person.