Category is one of the pre-defined taxonomies in WordPress. It is used to sort and group content into different sections. A website publishing content on a variety of topics can divide their website into sections using categories. Categories also make it easier for people to find your content.
Example: A news website can have categories for articles filed under News, Events, Sports, tutorials etc.
If no category is specified for a post then the post is automatically filed into the default category. In a new WordPress installation, the default category is ‘Uncategorized’. A website administrator can change default category from Settings » Writing screen. A WordPress post can be filed under multiple categories and can have both categories and tags.