Yes page content can of course be edited in the browser, this is one of the main features of a cms. An example of doing this is when you add a category/page in the cms you can then use the main description field to add/edit on page content.
To define/position the main editable content area in Xara, just create a placeholder object on your page and in the body html area put the special tag {page_content} - now when you edit a category/page in the cms that uses this page template you can use the main description field to edit the text in this area. Categories have one main content area that can be defined in a page template using this special tag {page_content} - in the next version of the cms it will be possible to define multiple content areas on a page template.
Updating content is all done in the browser and any users with admin access can login to the cms and update content. At the moment it's not possible to allow certain users to edit certain pages although this is something that will possibly be added in the future.