What is Drupal?
Drupal is a free open source software package that allows an individual or a community of users to easily publish, manage and organize a wide variety of content on a website. People with limited technology skills are enabled to run their own site, without the need of an outside developer or webmaster to run the system.

I totally share advomatic's viewpoint about why Drupal CMS is good choice to build a web-site:

Why do I like Drupal so much?

1. Don't re-invent the wheel: The web is a very big place, and there are many other sites out there trying to display content, control access, share information, and engage users all in ways similar to you. No need to start from scratch every time. There are thousands of modules in the Drupal repository. Chances are that the tool to do what you want has already been invented.
2. Momentum: By every conceivable metric (from the number of code contributors to the number of conference attendees), Drupal has more than doubled annually for the past 3-4 years.
3. The Open Source contribution model: There is no single Drupal company that develops the software, but rather a highly organized community of tens of thousands of developers and implementers around the world. If Advomatic fixes a bug, or extends a module's capabilities in some manner, we contribute those changes back to the Drupal community. The entire community benefits from each other's contributions.
4. Community: The benefits of contribution do not end at the bytes of code. Drupal is known for its strong local groups. There are over 500 members of the dozens of local developer groups in the New York City area alone.
5. Scalability and Performance: Drupal is optimized to perform extremely well with information repository sites. The modules allow your organization to add features to the site as your organization grows.
6. Customizability: Drupal assumes that each website requires information to displayed in a manner specific to that site. Drupal's flexible theme and module systems mean that everything can be manipulated to suit your particular needs.
7. Usability: Drupal has big plans for improving usability in its next release. By choosing Drupal you receive the benefit of several thorough usability studies that you wouldn't have with a custom application.