It could be just a different name for the same thing. A CMS is a "Content Management System", so it allows an administrator to edit the content (though, how much control they have over the content depends on the CMS).

However, an online page editor could simply be an online WYSIWYG tool for creating websites, which isn't necessarily the same thing. Something along the lines of http://ww w.websitew izard.com/ [remove spaces].

You'd really need to give an example of the online app or program that you are talking about to get a clearer answer.