A WikiPage is simply the basic building block of a WikiWikiWeb that contains text and images. See WikiStructures and WikiWikiWeb for more information.
Wiki pages can have an edit template to predefine initial content, see Cookbook:Edit Templates
Wiki pages are stored in individual flat files, see Page File Format and Flat File Advantages.
This page may have a more recent version on pmwiki.org(approve sites): PmWiki:WikiPage, and a talk page: PmWiki:WikiPage-Talk.