Most website design systems, like WordPress, include the ability to have posts as articles that are added to your site like pages, but also have a date and author assigned to them, along with topic categories and tags.
Categories and tags help your site visitors narrow down your site content by topics they are interested in. It’s like an instant pre-defined search of your content where the search results are pre-filtered by the categories and tags (topics) you choose.
Any post can have one or more categories and tags assigned to it. This is helpful for articles that cover numerous topics.
Categories are broader subjects that may include many different topics (tags). So, a site may have twelve categories, yet have hundreds of tags. The tags are very important because you may want to present your site visitors with a specific list of results on a certain topic without having the unrefined results produced by a typical search.
There are website widgets (modules) that let site visitors choose from a list of categories or tags. The category selection is usually a displayed list or drop-down list. The tag selection is generally a longer list sometimes displayed in a word cloud that shows the most common tags as larger bolder words.