Template:Popup

Description
This is a template for using custom Reference Popups in your pages. The template is somewhat complicated but should be easy enough to use. This template also has a sibling that you'll probably needs as well: ReferencePopups/PopupContent.

Simple inline popups: Blah blah ... ... blah blah

Complex inline popup: ... ...

This second form is useful because it allows you to avoid confusing MediaWiki and causing it to merge all your paragraphs together into a single colossal, unreadable wall of text. If you put  or   inline with the Popup template then the result can be very bad so you should use the detached form to do anything complicated like that.

Note
Unless you want to create your own custom template. You should copy this entire page onto your wiki as a template. ("Template:Popup" is recommended)