This is a set of comparisons I've made between Drupal and several other common CMS. I'm no expert in all of them, but having used each one for a period of time, I can at least give a good opinion on how they match and differ from the perspective of someone looking to make a website using one of these.

Drupal vs Wordpress

A common question and the answer is quite simple. Drupal is a full fledged CMS, Wordpress is a CMS designed for Blogging. Can Drupal do all that Wordpress can? Certainly. Can Wordpress do all that Drupal can? Nope. Then why use Wordpress? If your needs are purely to run a blog with maybe a few static pages. Meaning you don't intend run any forums, polls and so on, then use Wordpress. If you think that there might be a sudden time when you need to convert your web site into something more, like a promotional tool, or a community hangout, use Drupal. Also, Wordpress offers only basic search, while Drupal offers advanced search as well. This means you can search using boolean expressions like AND, OR, NOT and also restrict your search to a specific category and sub category if needed. Wordpress also limits categorization to single level with free tagging, meaning nested categories are not possible. Drupal allows for unlimited nesting of categories, relationships between categories as well as free tagging at all levels. Both have fully featured blogs with comments, permanent links and trackbacks. Drupal requires a separate module to be installed for trackbacks. Wordpress became popular after movable type went commercial. Wordpress allows easy importing from other existing online blog services Blogger, Livejournal, b2 etc. Drupal offers importing from some services, but not all. More can be expected in the future. People have also managed to automatically import a static HTML site easily into Drupal. Both Drupal and Wordpress now have support for AJAX in their latest versions.

Drupal vs PHP-Nuke

Like Wordpress, PHP-Nuke is also designed towards a specific purpose. PHP-Nuke is meant for community interaction and allows user contributed articles, polls and forums as a standard feature. It also offers a download section, individual user control panel and more. Drupal can do everything that PHP-Nuke does, but certain features require extra modules to be installed (e.g. Downloads). Despite the claims, PHP-Nuke is not suitable for personal sites and blogs. Those who have managed to use PHP-Nuke for those purposes have extensively hacked the code. Its better to spend that time on developing your site with another CMS that is more suited for the purpose you seek. PHP-Nuke also has a known history of security problems, but several other CMS with better security have been made based on PHP-Nuke (e.g. XOOPS, PostNuke, CPG-Nuke) I have seen several PHP-Nuke sites that have been hit by script kiddies and bear large "This site has been hacked" messages on the front. If you decide you like PHP-Nuke, at least use one of the derivatives like XOOPS or PostNuke rather than PHP-Nuke itself. Some users have also released patches for PHP-Nuke to make it more secure.

Drupal vs Mambo/ Joomla

This is another question several people, including myself have asked at one time or another. In reality both systems are very alike and target the same market. Both attempt to be the all in one CMS, trying to be suitable for every type of site. Joomla only offers up to two levels of classification unlike Drupal. It also does not support cross categorization and free tagging. In fact, no other CMS I've seen or tried can match Drupal's flexibility in categorizing content. For a visitor to be able to browse through your site easily and for a search engine to be able to index your content well good categorization is key. In Joomla, the content is closely tied to the system of organization. Every article created must belong to a section and a category. A category is a sub classification of a section. In Drupal, it is possible to create articles with no categories and categorize them later. In both CMS, the menu which is displayed to the user is independent of the way the content is classified in the database. Both have revisions, meaning when editing an article, older versions can be archived instead of being replaced by newer versions. Joomla offers a slightly easier way to install new components, but makes it harder to manage them later. Joomla components come in tarballs which are automatically untarred and installed by the system upon upload. Each component creates a folder in which several files exist. In Drupal, a module is simply a single PHP file, not a set of files. The file has a .module extension and can be edited with any PHP supporting IDE. In Joomla, several files may have to be edited at times to make changes. Joomla's search works right out of the box, Drupal needs cron to be configured to use search. An easier way is to use the poormanscron module or an external service like Webcron if your host does not support cron, which is highly unlikely anyway.

Joomla is marketed far better than Drupal. Since it displays a nicer looking installation, it is believed to be more user friendly. But Drupal does not even need an installation; its simply upload the database and run the program. If you are looking for an easy first time experience with no more than click and point, go with Joomla. But Drupal offers a much better long term investment especially if you need to make changes, upgrades and so on. Joomla also gives an option to install sample content so that the user can learn by editing an existing site rather than create a new site all by himself. Drupal does not install any content by default, but the development module, if separately downloaded and installed allows sample data to be quickly entered for development and testing.

Joomla is more bandwidth consuming than Drupal. Gzip compression must be used to achieve a reasonable performance. Both systems support caching and clean URLs. Drupal also supports much easier URL aliasing, which is necessary to make your pages more search engine and user friendly. URL aliasing can also help to manage broken links better by redirecting an existing alias name to a new piece of content.

Drupal has full support for AJAX as of version 4.7. This means that introducing more dynamic features like drag and drop into the web site will be quite easy.

Overall

I could carry on comparing Drupal versus tons of other CMS out there, but I think anyone reading this will get the general idea on what Drupal is good for. You can read the entire list of features, which can be extended anytime using modules. Drupal is also very scalable and very large sites supporting tens of thousands of users have been built on Drupal. Hence, Drupal is both a CMS as well as a CMF at the same time, which is quite unique.