What's the difference between Alt Tags and Image Text?

I recertly received this question from a friend of mine in the SEO Industry.

To answer the question both tags are geared to inform the visitor of information on the site. Alt tags have been recognized by Google as the more important of the two when it crawls the page. Both can still suffer from keyword stuffing so making sure the density is correct can still be vital.

Alt Tags: Display alternative (alt) text when the browser does not load a photo image for whatever reason, security levels, browser settings. Example: Picture of a Tree does not load it will say "Pine Wood Tree" to show that a picture of a "Pine Wood Tree" should be shown if the browser did load the image.

Image Text - Is a hover over statement about describing the image in greater detail. For example a picture of a Pine Wood Tree may have a hover over describing the picture, "Pine Wood Tree in Colorado Mountains"