The Content API client library is a lightweight tool, available for both the browser and Node.js.
The Content API is designed for any situation in which you want to fetch data from your Ghost install. It supports a wide range of use cases, such as creating a latest posts widget on your site or generating a fully static Gatsby site.
The Admin API client library is designed for integrations that use private API keys, and is for Node.js only.
The Admin API supports publishing workflows where you want to integrate Ghost with other tools, such as publishing content from a desktop editor, automatically sharing content to social media, or syncing data to and from email campaign software.