Alt text is quite simply the tags that tell what a .gif or .jpg image is about. It is considered by the search engines to be an important tag. While it may be not as important as the title, descriptions or keywords meta tag, it is still intelligent to utilize it correctly for SEO purposes. Primarily used by screen readers for the blind to let the user know what image is being presented on a web page, the alt text attribute is an effective way to include the primary keyphrase for a page.
Proper implementation of this tag can be a good way to help the search engines assign the keyword relevancy of image based navigation, to assist the visually impaired and to highlight the more important keyphrases on a page for the search engines. While not as important as it used to be, it is always good to cover your bases and ensure that there is a descriptive enough explanation of the image that also includes the primary keyphrase for the page. It’s the general consensus that text in the alt tag text should be no longer than 6-7 words long and be as relevant to the image as possible. Google itself has outlined the importance of alt text by saying they use the alt text, “to help determine the best image to return for a user’s query”. (http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=114016)
Alt text is also helpful in that proper implementation can increase the chances of an image of a website showing up in Google’s universal search. If someone were to search for “the best cleaning widget for carpets” and a website selling that has correctly optimized the alt text for a carpet cleaning widget, they have the opportunity to show up in the image search in Google. This gives anyone utilizing their website for ecommerce a better chance of getting some helpful traffic as anyone seeing an image of something they want to buy are more prone to get to the next step and visit the website.