As a developer, one of the first new concepts you're going to need to understand for Drupal 8 is PSR-4. Or, the PHP Framework Interoperability Groups (php-fig), Proposal for Standard Request number 4 (PSR-4). A lot of fancy naming, abbreviations and numbers for a pretty simple idea. Allow your PHP classes to be automatically discovered and loaded by following a convention for class namespace and file/directory naming.