HAL - Hypertext Application Language is a format you can use in your API that gives you a simple way of linking. It has two variants, JSON and XML.
HAL provides a set of conventions for expressing hyperlinks to, and embeddedness of, related resources - the rest of a HAL document is just plain old JSON or XML.
Instead of using linkless JSON/XML, or spending time developing a custom media type, you can just use HAL and focus on defining and documenting the link relations that direct your clients through your API.
HAL is a bit like HTML for machines, in that it is generic and designed to drive many different types of application. The difference is that HTML has features for presenting a graphical hypermedia interface to a ‘human actor’, whereas HAL is intended for presenting a machine hypertext interface to ‘automated actors’.
