Guzzle is PHP HTTP client that will make your life easier, code cleaner and more testable and other developers will understand it. Forget about drupal_http_request, file_get_contents, pure Curl API.

I will describe Guzzle structure and predefined plugins, difference with 3rd and 4th versions and why it was chosen for D8.

Then I will show power of Services Description.

Then will show how to use Guzzle with Drupal7, Drupal8, other frameworks like Symfony2.