Worpress is a popular open-source platform for blogs and Joomla is also open-source CMS and extremely popular system used for many websites. What is the difference between them and which is better? Since they are actually different systems with different philosophy it is very difficult to compare them and even harder if not impossible to decide which system is better.
Each website has some purpose and basic type of use. Based on this purpose you should make the decision which CMS to use. You should also know that there are also other content management systems like Drupal or custom solutions that can be made according to your specifications. One of the greatest advantages of using open-source CMS like Joomla or WordPress is huge community and many available extensions.
Both popular web systems have many things in common. There are thousands of templates or themes for WordPress and Joomla. Some are free, for other you have to pay some small fee. Another common property is the availability of a huge number of extensions. You can get a plugin (or component / module for Joomla) for every imaginable function or task. This means that you can make your website according to your wishes and needs. And if there is no extension you can still find someone who will create it for you.
Now you may ask what is the point of choosing between Joomla and WordPress if both systems can be used to create any kind of website. The answer is pretty simple. Joomla is a universal CMS. It is build on a general-purpose platform which allows easy adaptation according to your preferences. WordPress, on the other hand, is a blogging platform. This is the primary purpose of the WordPress CMS. You can still add plugins, theme and other customizations but the system will stay oriented towards blogs. Blog means posting articles where visitors can add comments. You can disable these blog features and make a "normal" website but administration user interface will stay the same.
Another difference is the time needed to get familiar with the system. WordPress is pretty simple even for beginners while Joomla needs some time to get familiar with the CMS concept. Therefore, it is very hard to compare Joomla with WordPress. The best approach, if you are not familiar with them, is to check them both with some test installation and then decide which is better for your website. Regardless of your choice you will use a system that has great support.