There are 3 headless offerings out of the box in Sitecore 9+:

**1) The Sitecore Item API** -
This has been available for quite some time. However it isn't always seen as True headless due to it not including presentation details or personalisation in the response.

https://doc.sitecore.net/sitecore_experience_platform/developing/developing_with_sitecore/sitecoreservicesclient/the_restful_api_for_the_itemservice

**2) JSS** -
This uses the Layout service (It has it's own individual JSON structure) to support Front-end Apps (e.g React, Angular, Vue) whilst maintaining full Sitecore editing and personaisation features. This is seen as a True headless offering. JSS requires a consumption based licence at time of writing.

https://jss.sitecore.net/

**3) SXA** -
SXA also uses the Layout Service (again with it's own individual JSON structure). This powers the SXA Components. SXA is available with a on-premise licence (at cost) as well as included in an consumption based licence (at time of writing).

https://doc.sitecore.net/sitecore_experience_accelerator/getting_started_with_sxa