In XWiki, everything is historized and saved under Version Control.

History View

See View the History of a Page.
See History Tab.

Major and Minor Versions

See Major and Minor Versions.
See Minor Edits.

Actions for Admins

See Delete Page Versions from History.

Comparing Versions

See Compare Versions of a Page.

Raw and Rendered Changes

See Compare Version View.

Show next/previous changes

See Compare Version View.

Viewing Previous Versions

See Previous Versions of the Page.

Blame View

When viewing the source of a page (by going to More Actions > View Source on a page), you can enable the blame view by clicking on Show last authors button. In this mode, each line of the page's content is annotated with the last author that modified it and the revision in which this modification was introduced. The blame view also works when viewing the source of a page revision (other than the latest one) so it can be accessed at any point in the history of a page.

Hovering the avatar and the revision number will display the author's name and the date and time when the line was changed. Multiple lines can be grouped to express the fact that they were all modified within the same revision (by the same author and at the same time).

Clicking on the revision number will display the changes view, with all the modifications performed in that particular change by its author.

At this point, if you still have not found the change you were looking for related to a particular line of content, you could continue the search by clicking on the parent (left) version, opening the blame view for it and repeating the cycle. This way you can drill down the history and focus on the content and find out, for example, when exactly was a line added, in which context and by whom.