You know, when you hold your mouse over an image for a while and that little text box comes up.
It's called a tooltip.
Correction: A tooltip is when a balloon appears and gives the user a bit of information about a task they are about to perform, traditionally associated with an "i" icon or "?" a question mark icon. Rollover or mouseover text is the text that appears for HTML hyperlinked text and images and was originally created for browsers that may be unable to publish photos and gives users context to what is being shown without seeing the image.