Link module overview
The Link module is a core module that only allows external links. Internal links can be added with the core module Entity Reference.
Installation: The Link module is part of Drupal core and can be enabled from the Extend page (admin/modules).
Input format: The Link module only allows external (i.e. absolute) links. Links can include anchors or query strings.
Validation: Links must include the web protocol (http:// or https://) in order to validate. Validation takes place after filling in the field. Other scheme names (e.g. ftp, git, etc.) are not supported.
Link texts: Link texts (called "Link titles" in D7) can be added to each URL. Whether this is allowed or required can be set in the field management. Placeholder text can be added to appear in the fields before the user adds their own content.
Display settings: allow displaying either the link text or the URL, and specifying the number of characters at which the link text should be trimmed.
rel="nofollow" option notifies search engines that the link should not be followed. (See Wikipedia for details.)