Wikipedia offers various tools to edit pages.
The visual editor is a rich-text editor. To open a page using the visual editor, click "Edit".
The experience using it is similar to using a text processor.
Some pages are not editable using the visual mode. You have to use the source code instead.
The content of the page you edit is saved as Source code.
The source code of Wikipedia is called "wikitext". To open a page using the source editor, click on "Edit source".
Any page is editable using the source code.
Some pages can't be edited, because they are protected. Read more about protections.