The Facebook Ads API (aka Marketing API) is used by application developers to create, manage and measure ad campaigns that run on Facebook.
The Facebook Marketing API (previously referred to as Ads API) is an interface into Facebook to create ads, manage campaigns, and measure the performance of a campaign. The Facebook Marketing API has various levels of access, some of which may require an application and review process. See Facebook's Marketing APIs access documentation for more details.
The Facebook Marketing API is a subset of the Facebook Graph API where requests and responses are sent over HTTP and the responses are usually returned as JSON.
The Facebook hosted documentation is available here: Facebook Marketing API Documentation