Foundations
HTTP
Also: HTTPS, web request
The request-and-reply language of the web: a client asks for something, a server answers with a status and content.
In more detail
A request names a method (such as GET), a path and headers; the reply carries a status code (200 worked, 404 not found, 403 not allowed) and a body. Most web vulnerabilities are about what a server does with the parts of a request an attacker controls. HTTPS is the same conversation inside an encrypted connection.