WordPress is an open source Content Management System. It is basically a user-friendly blog system but it can be used as a regular site that even if you don’t have any knowledge in HTML , you can manage a site on your own. What is great about WordPRess is that you can add plenty of functionalities in your site through plugins.
Joomla is also an open source CMS that is aimed at creating a community or a network with membership areas, forums, articles, newsroom etc. Joomla gives you a lot of power as you have plenty of options to customize. If you want to learn Joomla fast, you have to dig in to reading a book.
Here’s an infographic describing the difference between WordPress and Joomla: