How to monitor changes in one or more page
This page explains the options you have to follow what is changed in a MediaWiki like this one you are looking at right now.
Monitor all changes in the wiki
You can follow all changes in this wiki with the Recent changes page, linked to in the menu on the left.
Monitor changes on a single page
Wiki pages are versioned, and each page has a history. On the top of the page, along the tabs 'page' and 'edit' there is a page called 'history'. This will give you the full list of changes made to that particular page.