use of ALT tag in HTML

what is the use of ALT tag in HTML?
The required alt attribute specifies an alternate text for an image, if the image cannot be displayed.
                                         OR
The alt attribute provides alternative information for an image if a user for some reason cannot view it (because of slow connection, an error in the src attribute, or if the user uses a screen reader).


(Note- Internet Explorer also displays the value of the alt attribute as a tooltip when mousing over the image. This is NOT the correct behavior, according to the HTML specification.)


Tip- To create a tooltip for an image, use the title attribute!