Complex images contain substantial information – more than can be conveyed in a short phrase or sentence.
- graphs and charts, including flow charts and organizational charts;
- diagrams and illustrations where the page text relies on the user being able to understand the image.
In these cases, a two-part text alternative is required. The first part is the short description to identify the image and, where appropriate, indicate the location of the long description. The second part is the long description – a textual representation of the essential information conveyed by the image.
(taken from https://www.w3.org/WAI/tutorials/images/complex/)