What is Moodle ?
Moodle (moodle.org) is a "CMS", but it is not an ordinary content management system like other popular CMS, it is mostly known as "course management system" or "Virtual Learning Environment" (VLE) as it is focusing on providing a tool to educators to create effective online learning communities
Moodle is designed and built with pedagogical principles (docs.moodle.org/en/Philosophy), it is released under GPL and is free to download and use, the program is actively maintain by a long list of developers with educational background, that means the tools is built by people who knows what is needed for building a good tool.
Educators, teachers, students and many educational institution has selected moolde as the tool for building their online communities, sharing course material, making quiz and more. Moodle is ready in different languages to cater for people from different parts of the world. You can take a look at the statistics (moodle.org/stats/) to see how popular moodle is around the world.
Moodle official site is a typical example of one type of website that can be built using moodle, moodle is stable and scalable, it can easily accommodate thousands of users and does not require huge hosting resources.