If you use the Content Management System (CMS) I recommend, Webvanta, there is back up every night. They have servers (where your site files and images are hosted) in more than one location to make sure your data is safe.

There are other CMSs which also provide back up but often are not automatic.

If I develop a Website for you where there is no CMS involved and I’m the only one making changes to it, I back up the site when it is initially completed and then every time I make changes.