PRODUCT: API CMS
Contentful is a content-as-a-service platform based around a content API. It's quite like a CMS, because it's ultimately used for managing content, but very much unlike WordPress, Drupal, Joomla and others of the kind: Contentful expects you to bring your own custom front-end. You still would create, manage and structure your content, but you wouldn't set up its appearance directly in the CMS.
Contentful is optimized for (and loved by) developers: indeed, they are the ones who build websites an apps which display the content. Using the API, developers request content from Contentful, and then they're free to do whatever is necessary with it.
The advantage of this approach is that the process becomes much more flexible: while editors still enjoy a familiar way of creating content, developers can easily plug it into their projects, since the content becomes structured and code-friendly.
The content-as-a-service approach means that there's no tech infrastructure to set up and maintain: your content is always available in the Contentful cloud, and you can simply use it.
Contentful is used by a number of well-established companies, such as Specialized, Jack in the Box, Nasty Gal, and many others.