When browser vendors bend the standards and implement something in a different way than what the specification states, they may cause problems, or at least confusion. One example of this is the way certain browsers, the most widely used being Internet Explorer for Windows, handle alt attributes (popularly
and incorrectly referred to as “alt tags”).

Alternate text is not meant to be used as a tool tip, or more specifically, to provide additional information about an image. The title attribute, on the
other hand, is meant to provide additional information about an element. That information is displayed as a tooltip by most graphical browsers, though
manufacturers are free to render title text in other ways.

Read more at
http://www.456bereastreet.com/archive/200412/the_alt_and_title_attributes/