The Migrate API can be used to import content from an older Drupal site (Drupal 7 or older), or any other data source, into the current version of Drupal.
Themes provide the HTML, CSS, JavaScript, and other assets that are responsible for the look and feel of your site.
The Render API consists of a standard format for describing data using structured arrays and a process for converting those arrays into the HTML a user sees when interacting with a Drupal site.
Drupal Core Modules
TopicA module is a set of PHP, JavaScript, and/or CSS files that extends site features and adds functionality. The Drupal core software comes with a set of modules that provide a range of features.
Hands-On Theming
GuideTheming Cheat Sheet
GuideLearn Drupal
GuideIntroduction to Drupal
GuideDrupal User Guide
GuideDependency Injection
TopicDependency injection is a design pattern commonly used in object-oriented software architectures in order to support Inversion of Control.
Development Environments
TopicDevelopment environments provide a sandbox where you can work on your application without affecting the live site.
Layouts
TopicA layout can describe how various components are arranged on various levels—from an entire page from the header to the footer, to just the “middle” where the dynamic content goes, to individual components. It can apply to templates for managed content or one-off designs for landing pages.