The alt attribute is designed to be an alternative text description for images. alt text displays before the image is loaded. It is a required element for images and can only be used for image tags.
In contrast, the title attribute can be used for any page element, but it isn’t required for any page element. Many search engine ranking algorithms read the text in title attributes as regular page content.
In the past, the alt text description was often used as a sort of pop-up tooltip on images. That was because the title attribute wasn’t widely supported. Today most browsers interpret the W3C specifications rather strictly – alt text no longer “pops up” when you run your mouse over an image. You have to use a title attribute for that.