Good content is relevant, timely, interesting, and worth sharing with others. Content comes in many forms. It can be in the form of:
Articles
Videos
Images
Flash movies
Comments
Forum posts and much more
It's also the headlines, title, subtitles, and other helper text surrounding it.
At this point in the world of search engines, content is textual. Even if you create videos or images that are high quality, the search engines use the words—the text—around those media to decide what they are and how to categorize them. This means that the written word is far more important to search engines than any other kind of media.
We'll cover video and images a bit later. For now, let's talk about writing.