Search engines have developed a lot of sophisticated techniques for weighting and valuing pages on the Web. But they all come down to basically two categories:
When you look at a Web page, you see the page displayed on your computer screen. You can read the text, look at the images, and figure out what that page is about.
Search engines don't see Web pages the same way a person does. In fact, search engines cannot actually see at all, at least not visually. Instead, they read the HTML code of the Web page, and the actual text that it contains.
All the search engines can read is text. They also can look at the HTML code (which is also text) of the site to try and get some clues about what that text means or which text is most important.
Search engines can sometimes use the HTML code to get some clues about other elements on the page, such as images and animation. For example, search engines can look at an image tag and read the
alt text attribute, if the page author supplied it, to get an idea of what the image is.
img src="cowpicture.jpg" alt="Picture of a cow"
However, this is not a replacement for actual text content.
Web links from other sites are also important clues that search engines use to figure out what your page is about, or how important your page is for a particular search query.In a search engine's view, a link from one page to another is basically a "vote" for that page.
If you have a page about cows, and a local farmer's Web page links to your page from their website for more information on the topic of cows, that is an extra vote for your page.
More links = more votes.
Not all votes are equal votes, however. Most important is how relevant the link is. For example, a link from a page about video poker software doesn't have much to do with dairy products or cows, so a link from that page to your website about cows does not count for very much at all, if anything.
Some Web page owners put a lot of time and effort into chasing down links from other Web page authors, swapping links or trying to get listed on directories or have articles posted to sites like Digg or Reddit. This can be helpful for your site, but you have to remember to focus on your own page content first. If your Web page doesn't have much value to other site authors, they are unlikely to link to it.