Alt Text - SEO Glossary
Alt text is the text that gets displayed when images do not load on a web page, this is a feature used to make web pages more accessible to people with diabilities. People often stuff the alt text of images hoping that this will increase their rankings in search engines, this will not help them a great deal as its purpose is mainly for other accessability issues. Having said that it is worth while writing in at least one keyword in the description if possible.
Microsoft Internet Explorer incorrectly uses the alt text attribute as it displays the text when moving the mouse over an image even when the image is present, instead what it should show is the title text if it is present and not the alt text - this should only be shown when the image does not load for one reason or another.