WordPress started out as a blogging platform. It first had only one type of content entries called Posts. By default, posts are based on dates like news or entries in a diary.
As WordPress grew beyond simple blogs its developers realized they need to provide another type of content. This is when they created Pages. A page is not bound to a specific date and its content is usually relevant for a longer time, like an “About Us” page, for example.
In WordPress, Posts and Pages are the built-in post types. You can also create your own, custom post types for literally any type of content you want.
Post and Pages have some standard fields for storing main information: title, main content, date, author, and others. However, as WordPress evolved, it added the possibility of creating custom fields for posts. You can create your own custom fields to add literally any type of additional information to your posts.
Finally, to help you organize your content, WordPress allows you to categorize it by using taxonomies. Blog posts provide you with Tags and Categories, the two built-in taxonomies. You can also create your own custom taxonomies.