System requirements

Disclaimer
Note that the minimum requirements of Drupal core can be subject to change, given an important enough reason. Where such a change is likely to have broad impact on Drupal’s user base, every effort will be made to communicate these decisions ahead of time.

Disk space
A minimum installation requires 15 Megabytes. 60 MB is needed for a website with many contributed modules and themes installed. Keep in mind you need much more for the database, files uploaded by the users, media, backups and other files.

Web server
Apache, Nginx, or Microsoft IIS
More details about web server requirements.

Database
Drupal 6: MySQL 4.1 or higher, PostgreSQL 7.1,
Drupal 7: MySQL 5.0.15 or higher with PDO, PostgreSQL 8.3 or higher with PDO, SQLite 3.3.7 or higher
Drupal 8: MySQL 5.5.3/MariaDB 5.5.20/Percona Server 5.5.8 or higher with PDO and an InnoDB-compatible primary storage engine, PostgreSQL 8.3 or higher with PDO, SQLite 3.4.2 or higher
Microsoft SQL Server and Oracle are supported by additional modules.
More details about database requirements.

PHP
Drupal 6: PHP 5.x only (5.2.5 or higher recommended). Warning: support for PHP 4.x has been dropped. Drupal core should work with PHP 5.3.x, but PHP 5.3.x and higher may produce errors or unexpected behavior especially for contributed modules and themes.
Drupal 7: PHP 5.2.5 or higher (5.4 or higher recommended).
Drupal 8: PHP 5.5.9 or higher