The difference between bootstrap and a CMS is that bootstrap doesn't mess with any wysiwyg interface bullshit. It's just a pre-fabricated way to architect your html, css, and javascript, and the default styling makes things look very professional.
ie: you could still use Bootstrap to style a WordPress site.
Right, a CMS lets you implement an architecture for HTML and CSS, e.g. Bootstrap.
CMSs manage your content usually through a database, and most of them provide a way to display that content in a structured format. See Wordpress and Drupal. WYSIWYG is not a necessary part of a CMS.
Bootstrap is a (mostly) css library to help provide a better front end development experience for a web site/app. It can be used with the front-facing portion of a CMS or any other kind of site.