Ultimately, express uses node's http api behind the scenes.
express framework
The express framework provides an abstraction layer above the vanilla http module to make handling web traffic and APIs a little easier. There's also tons of middleware available for express (and express-like) frameworks to complete common tasks such as: CORS, XSRF, POST parsing, cookies etc.
http api
The http api is very simple and is used to to setup and manage incoming/outgoing ,HTTP connections. Node does most of the heavy lifting here but it does provide things you'll commonly see throughout most node web framework such as: request/response objects etc.