"Adopting various Symfony components has had a huge impact on Drupal, however people should not confuse the parts that are done with Symfony versus the parts that are done with PHP 5.3. PHP 4 and PHP 5.3 are different languages: conceptually, structurally; they happen to be mostly backward-compatible, but that's about it. Drupal 8's biggest shift is that it is a PHP 5.3 system. We have ported Drupal to a new language. That is the biggest jump for people. Symfony is a PHP 5.3 component library that we're using a lot of pieces of to save time and because they have already solved a lot of hard problems in ways that are very good."
Recaping some of the mentioned concepts I decided to curate this list including a summary for some of those concepts and links for more information and reference.
Composer is a tool for dependency management in PHP. It allows you to declare the dependent libraries your project needs and it will install them in your project for you.
YAML is a human-readable data serialization format.
Namespaces are a way of encapsulating items. Bringing the ability to organize and group related clases and also to avoid name collision.
The Use operator bring the ability to refer to an external fully qualified name with an alias, or importing. This is similar to the ability of unix-based filesystems to create symbolic links to a file or to a directory.
Dependency Injection is where objects or components dependencies are passed to another object using constructor or methods.
A Service Container (or dependency injection container) is simply a PHP object that manages the instantiation of services.
An anotation is metadata attached to your code using "docblocks", that can be read at runtime.
A route is a map from a URL path to a controller.
A controller is a PHP
function method you create that takes information from the HTTP request and constructs and returns an HTTP response.
Links & resources
Test Drupal8 online