What are categories in WordPress?

As a beginner there are some question about WordPress such as what is the categories....

So a categories are use for groping content on a WordPress site. It symbolizes a topic or a group of topics that are connected to one another in same way.

A post can belong to many categories at the same time however, it's perhaps not the best idea to assign more than two or three categories to one post. The point here is to have your content neatly organized in a way that makes accessing it easier for the reader.

Yes.

Basically a subdivision of your main topic/keyword

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Normally category is a some common division for your post. For example if your post is about mobiles then it will be selected as mobiles or smartphones category.
If you want to give more than one specific nane for your post then put it under the tags section.