REST APIs rely on the HTTP protocol to transmit data between the client and server. When a client makes a request to a server, the request contains information about the desired action and any necessary data. The server then responds with the appropriate information or status code, indicating the success or failure of the request. 

HTTP requests and responses consist of a header and a body. The header contains metadata about the request or response, such as the content type or the HTTP method used. The body contains the actual data being transmitted, such as a JSON object or a file. 

HTTP headers and content types play an important role in formatting data in REST APIs. Headers allow clients and servers to communicate important information about the data being transmitted, such as its format or encoding. Content types, in particular, specify the format of the data being transmitted, such as JSON, XML, or plain text. By adhering to a standard content type, developers can ensure that their APIs are compatible with a wide range of clients and servers.