I would like to take a few moments to put out this brief comparison review. I still come across people who say they've tried Joomla! and it was too confusing for them. When pressed on when they tested Joomla!, it was back in the early days, back with Joomla! 1.0 in 2005/2006. How can you actually make an honest opinion on something you checked out 10 years ago, but haven’t touched since?
Joomla! is completely Open Source software, from the beginning to the last piece of code. Joomla! has 7,825 (50% or so commercial) extensions which are actively maintained. All extensions on the Joomla! Extensions Directory are Open Source, otherwise they don’t get listed. A common thing that will be seen with other reviews is that WordPress has over 45,000 plugins, compared to the 7,800 for Joomla!, so the reviewers will claim that that shows Joomla! is "dying," which is false. Joomla! actually has the most vibrant community of all the open source CMSes (WordPress, Joomla, and Drupal).
The other thing that Joomla! does, where WordPress doesn’t (from my observations and research), are that any submissions to the Joomla! extensions directory go through a rigorous review of the code to ensure it meets the Joomla! standards, and also checks the code for common vulnerabilities. Additionally, if an extension later is found to have a vulnerability, or hasn’t been maintained to the current version of Joomla!, that extension is removed from the extension directory. This process keeps the extension directory fresh, accurate, and secure for your website.
Here is a compiled review by what is, in general, the "real" CMS reviewers (CMSexp0/CMSwire/CMSwatch/to mention a few) whom have written about what the three mainstream CMSes offer (most of the reviews you will find out there are written by those who actually admit to being pro WordPress, and you can tell by their review that they were incapable of being non-biased, therefore being an unprofessional honest review):
We have three major CMS systems in the world: Drupal, and Joomla!, and WordPress.
Drupal is the powerhouse for Developers and is not suitable for end-users with limited knowledge.
Joomla! can be used for very complex and multi-functional websites, and someone wouldn’t be able to create a fully versatile and integrated website with WordPress as you can do with Joomla!.
WordPress is very good and favorable if you want a blog page or a news site. Most of the WordPress plugins have been developed for these kind of sites, where Joomla! offers full blown software systems for any site with its over 7,800 extensions (see the Joomla! Extensions Directory).
If Drupal is a powerhouse framework for developers, and WordPress is the world's most popular blogging platform, then Joomla is the platform that reaches out to developers, end users, and designers.
Joomla is flexible, customizable, and easy to use for simple web usage after no more than about one - two hours of training. Think of it this way: bicycle, car, airplane. WordPress, the bicycle, is simple, requires very little maintenance, and is great for the beginner. Joomla! is like a car, which is more powerful and functional than a bicycle, and with a little training you can drive it. Drupal is like the airplane, where it takes much more specialized training to understand and maneuver.
Previous versions of Joomla! have leaned on an early legacy of a blogging tool, making some concepts counter-intuitive for modern websites. But with Joomla 2.5. and beyond, now with J3.5, the CMS has improved in leaps and bounds, and now provides a solid platform for building small, medium, and large websites.
Powerful extensions exist that encompass features such as permissions-based file repositories capable of handling tens of thousands of files (like DocMan), integration with social networks, membership management (like Payplans Pro), email and sms marketing (like Acymailing and AcySMS), ecommerce (like EShop), and community based social networks (like EasySocial and Community Builder).
The roadmap for Joomla! shows that they are working towards a lean and very flexible, but still powerful, CMS. Just for the record: The Joomla! help-screens in the administrator back-end are part of the extensive Documentation Portal that can be reached through this link: https://docs.joomla.org . Joomla! has a huge and very vibrant community and a very active support forum.