System requirements

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
Microsoft SQL Server and Oracle are supported by additional modules.
More details about database requirements.

PHP
Drupal 6: PHP 4.4.0 or higher (5.2 recommended).
Drupal 7: PHP 5.2.5 or higher (5.3 recommended).
Drupal 8: PHP 5.3.10 or higher.

Memory:
Drupal 6: 126 MB and more.
Drupal 7: 256 MB and more.