Drupal is a hybrid and somewhat atypical CMS, released under free license and distributed free. Its modular operation makes it possible to create a wide variety of websites: from the personal blog to a company's website, via community websites, news sites, corporate intranets, online shops, etc. Drupal combines the advantages of an excellent CMS (simplicity and efficiency) with those of a framework (modularity and scalability).
Drush is an abbreviation for "Drupal shell" - it is a command-line utility for Drupal. It allows a user to execute commands faster than in the Drupal administration interface (or not available by default). For example, it is faster to launch a text command to empty the Drupal cache than to access the various interfaces of the administration of your site!
A Drupal distribution is a set of modules, themes and configurations to set up a site with a specific purpose. There can be several such as Dkan, Roomify, Drupal Commerce etc ... Their goal is to meet the expectations of a web project quickly.
DKAN is a Drupal distribution that provides a complete and integrated management solution for an open data portal, designed to have features compatible with CKAN. Its name is of course a reference to CKAN, but it is a completely independent solution although interoperability is possible. DKAN has a different approach to CKAN, because it integrates the functionality of an open data portal into an existing CMS. Datasets are treated as content, offering rich workflows.
Open Data: Drupal / CKAN vs. Drupal DKAN
The technical debt is complex to explain. It can be defined as the accumulation of the development time needed to correct the code quality deficit (debugging, maintenance, redevelopment of certain pieces of code or entire functionalities, etc.) - To go further: The technical debt a Drupal project
This term refers to the core of Drupal that contains the essential features. With Drupal 8, each version (8.x) has new features. As the API is compatible from one version to another, it becomes possible to upgrade and benefit from these improvements quickly.
The Drupal core needs to be updated regularly to avoid security breaches. For this, we must follow the "Security advisories" on drupal.org/security