How is an issue linked in JIRA?

As the name itself defines, Linking means the association between the two.
 
In the same way, in JIRA issue is linked with any other issue in cases like :
 
* Relate to another issue
* Duplicate to another issue
* Block another issue.

For displaying the details of the Linked issues, there are two fields available in JIRA : ‘Linked issues’ and ‘Issues’.
Linked Issues
Linked issue section contains the dropdown for the options to be selected as the reason for linking the issue. As per the selected option, the suggestion of the issues to be linked in displayed in ‘Issue’ dropdown.
Linked Issues
Linking of issues can be done either on the same or different JIRA servers.