According to Google.com,
It's a good definition, but let's apply it to you. Marketing is all about breaking the huge mass of people on this planet into smaller groups then finding ways to reach those people. As an author, you are first looking for the group of people who reads books. That is a very broad market. To be effective in your marketing, you need to break it down further. If you write mysteries, you are looking for people who read mysteries. You can break your target market down even further. If you write cozy mysteries, you are looking for people who read mysteries, but prefer stories a step away from the gore.
It is easy to use a shot gun approach to marketing. Spatter your message to the world and hope that you might reach someone who is interested. Unfortunately, that method requires a huge marketing budget to be anywhere close to effective. Your budget is precious! If you're writing children's books about a fluffy bunny, don't spend time marketing to the death metal scene. If you write about Wiccan fun, find groups that aren't fundamental Christian.
To sum up: Who wants what you have to offer? That is your target market.