Modules are software plug-ins that let you increase the functions of your Drupal platform. There are thousands of modules available for Drupal users within the platform itself.
You can view your default list of modules at 'admin/build/modules' which can be enabled in one click. Some of the pre-installed modules are
Forum: It is useful to maintain threaded discussions.
Upload: It allows for attaching of media files to your content and uploading them.
Aggregator: It allows you to set up feeds like RSS to your website.
Poll: It lets you set up polls for public opinions.
Blog API: It lets you set up and maintain a blog through accepted standards.
Throttle: It allows your system respond better in times of higher website traffic.