Hypertext Transfer Protocol (HTTP) is the foundation of communication on the WWW. It defines the format of messages and the way they are transmitted between the client and the server.

HTTP has two sides to communication — the client and the server. On web applications, the client is the user's web browser and the server is the backend server. Communication happens when the client creates a message called an HTTP request, sends it to the server, and the server, in turn, responds with an HTTP response, as shown in the following diagram:

Each HTTP request consists of a few pieces of information:

• URI: The web address or IP address that identifies the server. For example, http://example.com or http://192.168.0.1.
• Method: The action that should be taken upon the requested resource. For example, GET indicates data retrieval and POST indicates data creation.
• Body: Contains the data to be sent to the server, if any. The most popular data format for HTTP requests is JSON but, generally, data can be formatted in any format that the server understands.
• Headers: An optional set of metadata key-value pairs that add information to a request.

The server retrieves this request, executes related logical steps (commonly known as business logic), and forms an HTTP response to be sent back to the client. This response includes a few details:

• Status code: A three-digit integer that describes the result of the request. For example, 200 means OK, 500 means there was an error in the server, and 404 means the requested resource was not found.
• Body: The data sent back from the server, if any. As with the request body, JSON is the common data format, but any other format is valid as well.
• Headers: An optional set of metadata key-value pairs that add information about the response.