Documentation for JIRA 5.0. Documentation for other versions of JIRA is available too.
To provide online help for a custom field, use HTML or Javascript in the field's description. E.g. you can have a simple link to an external help page:
<a href="http://www.mycompany.com/jirahelp/fieldhelp.html">get help</a>
Or using Javascript, you can have help text right in the field:
where clicking the help icon makes hidden help text appear:
This can be done by entering the following as the field's description:
Quality Assurance contact <script type="text/javascript"> function showHelp() { var listenersDiv = document.getElementById("qaFieldHelp"); if (listenersDiv.style.display == 'none') { listenersDiv.style.display = ''; } else { listenersDiv.style.display='none'; } } </script> <a href="#" onclick="showHelp(); return false;"><img src="/images/icons/ico_help.png"/></a> <div id="qaFieldHelp" style="display:none"> The QA Contact is a member of the QA department responsible for taking this issue through testing. They will be notified by email of this and subsequent issue state transitions. </div>
(Incidentally, Javascript in descriptions can also be used to set field values.)