Successful HTTP API responses can return any media type, but the most common for modern HTTP APIs is application/json. For older APIs you may see XML, and depending on the resources and capabilities being made available you may seem PDF, CSV, other more specialized media type. All media types should be well defined and expressed as part of successful API responses.
Media types are how web responses are negotiated globally, and HTTP do the same, but for a more select set of media types that describe the shape of data, documents, and other digital resources or capabilities. HTTP API media types build on the web and increase the size of the audience who will be able to understand and process API responses in an automated way.