The Web Linking spec, RFC5988, as has been pointed out in another answer, defines some different types of link relationships. But it also instructs IANA to create a link relation registry and to allow further link relation registrations. That registry, which is the definitive list of public link relations, is available at iana.org/assignments/link-relations and will be updated as new relationships are registered.
Commonly used relations in HTTP APIs include:
start (points from every resource back to the API start point)item (points from a collection to an item, e.g. from a Twitter user page to a tweet)collection (reverse of item)previous (these next four are for paginated resources, e.g. collections or multi-page articles)nextfirstlastcreate-form (points from a collection to a resource that describes how to create new collection items, e.g. a ‘New Item’ HTML or XForms form)edit-form (points from an item to a form for editing that item, e.g. an Edit Tweet button)
If your desired relation is not covered by anything on that list, your relation must be a URI. Further, it is advised to make that URI a dereferencable http URL at a domain under your control so that API clients can look up documentation for the relation, e.g. http://www.example.com/link-relations#tweets. Usually, your API start point will be a list of collections, each with a custom link relation that describes what type of resource each collection contains.
