The Article Editor, has several buttons at the top that allow you to save your work:

• The Save & Close button allows you save your work, exit the Article Editor, and return to the Article Manager.
• You can also click the Save button at any time in order to save your edits to date, but to remain in the Article Editor.
• The Save as Copy button allows you to save the edited contents as a completely new article, preserving the old contents of the artile you were editing from.
• The Save & New button saves the page you are currently working on, but remains in the Article Editor with an empty new page. See the guidance on creating a new page.
• The Close button allows you to quit the Article Editor  and return to the Article Manager. BEWARE that any edits you have done since you last clicked a Save button will be lost.

Joomla maintains a number of Versions of each article, so it is very easy, using the Versions button, to return to a previous version, rejecting recent edits.