HTTP stands for Hypertext Transfer Protocol. The very first version was 0.9; then came version 1.0.
The key difference between 1.0 and 1.1 is that the client makes a connection with the server and that connection is reused, whereas in HTTP 1.0, the connection is thrown away and for each request, a new connection is created. HTTP 1.1 is also derived by applying the REST constraints to 1.0.
A basic HTTP message is composed of a header and body.
When the client communicates with the server, it communicates via HTTP. The server responds to the client with messages and code.
