Hyperlinks, or simply, links, are the building blocks of any website, as they help to create a relationship between different resources (e.g. webpages).
A backlink for a given web resource is a link from any given website (the referrer) to that web resource (the referent).
An iframe allows a certain web page to load content from another source without forcing the user to navigate to that specific page.
If you need to link to the following EU institutions' websites - Eur-Lex, EU-Bookshop, or Tender Online Daily - you must follow the rules on linking, as outlined here.