Drupal - Overview

Drupal is often described as a "content management system" (CMS) it is also a "content management framework". In other words, unlike a typical CMS, it is geared more towards configurability and customization. Picture a range of measurement where the one end of the scale is labeled “specific” and the other end “abstract”. On the “specific” end of the spectrum, you would have something whose form is very specialized because it’s meant for a specific purpose – like, say, a hammer. On the other end of the spectrum, you would have something much more abstracted, that is available to be configured any way you like, for a variety of purposes – like some wood and a chunk of steel. You could make a hammer, or any number of other things with the wood and steel.
Drupal’s purpose is to sit in the sweet spot between the two ends of the scale, and create a sort of “builder’s kit” made up of pre-designed components that can be used as-is or can be extensively reconfigured to suit your needs. Its design provides incredible flexibility while still allowing people who aren’t programmers to make powerful websites. This principle of manageable abstraction is important to understand, because it is a central concept to all things Drupal. When you understand why a measured amount of abstraction is valuable, you’ll begin to understand why this approach is such a strong argument for using Drupal. (from www.drupal.org)

Drupal Features

Collaborative Book - Drupal's unique collaborative book feature lets you setup a "book" and then authorize other individuals to contribute content
Friendly URLs - Drupal uses Apache's mod_rewrite to enable customizable URLs that are both user and search engine friendly.
Modules - The Drupal community has contributed many modules which provide functionality that extend Drupal core.
Online help - Like many Open Source projects, we can't say that our online help is perfect but have built a robust online help system built into the core help text. Available to you on your own site
Open source - The source code of Drupal is freely available under the terms of the GNU General Public License 2 (GPL). Unlike proprietary blogging or content management systems, Drupal's feature set is fully available to extend or customize as needed
Personalization - A robust personalization environment is at the core of Drupal. Both the content and the presentation can be individualized based on user-defined preferences. Role based permission system - Drupal administrators don't have to tediously setup permissions for each user. Instead, they assign permissions to roles and then group like users into a role group.screenshot
Searching - All content in Drupal is fully indexed and searchable at all times if you take advantage of the built in search module. (find out more: www.drupal.org)

Drupal recourse website and company who use Drupal CMS:

lullabot.com
acquia.com
Amnesty International - amnesty.org
The Onion - theonion.com
Populat Science - popsci.com

When NOT to use Drupal?

When it's overkill:
sites what won't change much;
sites that don't take advantage of its features;
when solid, simpler alternatives exist - joomla, Wordpress, LiveJournal for blodding, MediaWiki for wikis;

When you don't hame technical help or abilities
Very high-traffic or mission-critical sites