Drupal is part of a technology stack that contains a number of important pieces.
A server is a computer which provides information or services to other computers on a network.
The operating system is the software that runs the server. Unix, Linux, BSD, OS X and Windows are some examples.
A database is a structured collection of records. Drupal uses a database to store most content and configuration settings for your site, some content such as media files are generally stored in the server's file system. Examples are MySQL and PostgreSQL.
The web server is the software component responsible for serving web pages. Examples are Apache and Microsoft IIS.
PHP is a programming language that allows web developers to create dynamic content that interacts with databases. To communicate with the databases the neceessary database drivers (e.g., PDO_MYSQL) must be enabled. To process images certain libraries (e.g., GD) must be enabled as well.
Drupal is a framework for building dynamic web sites offering a broad range of features and services including user administration, publishing workflow, discussion capabilities, news aggregation, metadata functionalities using controlled vocabularies and XML publishing for content sharing purposes. A Drupal installation is generally comprised of a mix of core and contributed modules.
In the requirements section you find details about all the elements of this stack.