There are 2 basic types of Alternative Text:
An Alt attribute, sometimes referred to as an "Alt Tag", is a way to store brief alternative text descriptions for an image. Even though there is no limit on the number of characters allowed in an Alt attribute, they are usually only 8-10 words or less. The Alt attribute is stored directly in the HTML code in the web page.
The Long Description attribute is a way to store alternative text descriptions that are usually longer than 8-10 words. The Long Description is stored in an external file that users can access when needed.
Sometimes an image does not need to have a textual description because the image does not convey any information. The image might simply be for decoration. A null string tells assistive technologies like screen readers to ignore the image. Both the Alt Attribute and the Long Description can be assigned a null string.