Extra blocks on your pages: in WordPress they are called widgets, in Joomla modules. For example, you can use them to show content in a certain way.
By default, a few widgets are present and activated in WordPress, such as those for most recent posts, recent comments, search, and archive. So basically widgets aimed at blogs. In addition, standard widgets are installed with which you can display, for example, one or more images, a tag cloud or a calendar of your blogs. You can't set as many in it, usually it is limited to the number of items you want to display.
Joomla comes with many more module types as standard. There are more variations for content alone, such as the most popular or most read articles. But modules in Joomla go further than just showing content: for example, you also display a menu or a login form via a module.
The modules in Joomla have more settings than the WordPress widgets. Content modules, for example, have extensive filtering and sorting options. Also handy: in Joomla you can indicate to which user group your module should be shown.
Where to put them?
Joomla modules are a lot more flexible than the WordPress widgets. If you add a widget to your theme in WordPress, it will be displayed in that place throughout the website. So you can't say: don't show this widget here. At Joomla you can determine per module whether it should be shown and where.
In the standard WordPress themes, the number of places where you can place a widget is limited to (usually) footer and sidebar. The standard supplied Joomla templates have at least six module positions.
A second method to display modules or widgets is to put them in your page or article. This option is similar in both systems: Joomla has a button to insert a module, WordPress has a widget block in Gutenberg.