What do you know about your website's audience? How well do you understand them? As with any communications medium, the web requires authors to have a good understanding about those with whom they wish to communicate.
The web offers a myriad of choices when it comes to communicating with your audience. You have to make visual design choices, technology choices, information architecture choices, and all of this before you've even written a word of content. Different audiences will have different likes and dislikes within each of these areas so it's important to understand who it is you want to reach most. Understanding your audience will be of great benefit when you begin making design decisions for your website.
The web is a vast information space that many different people use. In order for your site to be effective, it must be able to deliver its message to some of those people. In order to do that, we have to narrow down who we're trying to reach. We can do this many ways, but one of the more effective methods is to start by identifying your primary audience.
Your primary audience is composed of all of the different groups of people that you are specifically targeting with your site. These are the people you must reach in order for your website to be successful. Identifying the groups that form your primary audience may or may not be difficult, but even when the answer seems obvious, it still requires thoughtful reflection to make certain you've not excluded anyone and that the primary audience is defined as specifically as possible.
If you're developing a site for someone else, then they will have to tell you who their primary audience is. You may have to ask and re-ask though to make sure that they've thought the question through. If your web site is for your own business, your primary audience should already be detailed in your business plan. Finally, if your web site is non-commercial and your own creation, you'll need to rely on your own intuition and some thoughtful reflection to pin down your primary audience. You'll have to ask yourself questions about who exactly you're trying to reach with your site. How old are they? What language do they speak? Where do they live? What is it that your site is targeting that they share in common? What else might they share in common?