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.
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.
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