When a search engine spider visits a site it is only able to see the text that appears on the page. Any images, Flash elements, or dynamic content (JavaScript etc.), is almost certainly not visible to the spider. Often what a human can see on a screen as text is just an image, to the spider it will be meaningless.

As an example the following text is actually an image:

Spiders can't read this :-(

You and I can read it however to a search spider it is just an image, the same as a photo or diagram. There are many valid reasons why your site may include text as an image. The main reason being that web text is limited to a small number of fonts, using images allows for any font available to the designer.

This inability to index non-text content has dramatic consequences for the ranking of your site. No matter how relevant the content is if the search engine cannot see it then it cannot be indexed. Good examples of this are all Flash sites, for some of these there is absolutely no content that a search engine can see. Imagine what that does to their ranking!

This does not necessarily have to be a problem as there are ways for the web developer to inform the search spider about content that is contained in these non-text elements. For example within the HTML for an image a developer can specify what is called the "alt" attribute which tells the spider the text that should be assigned to the image.

It should be noted that search engines, in particular Google, are working on ways to index non-text content however it is early days and the results are inconsistent and significantly behind the indexing of general text.

If you would like to see how a search engine sees your site please check out the spider simulator.