The visual editor is a feature that aims to prevent new users from wiki markup it is restricted to the namespaces User, File, Category, (Main).
Sometimes, especially when the pages have structural issues, the visual editor is doing things you are not aware of. This may be confusing to others because your name is stored in the article history along with this change. When editing a page in the visual editor, it's possible to switch to the plain text editor that is showing you the actual wiki markup.
The visual editor is enabled by default. It can be enabled/disabled in Preferences → Editing.
In the article history and the recent changes list, you'll see changes with the visual editor indicated by special tags:
- The changes was made entirely with the visual editor
- VisualEdit: Switched
- The user started editing with the visual editor than changed to the wikitext editor
This makes sense because some edits cannot be done (or done the right way) outside the wikitext editor.