Technically, everything in WordPress is a Post. Post is the default term for a content item.
The default Post Type in WordPress is called “Posts”. As WordPress began as a blogging platform, we predominantly use this for news, and rename it in the Back-end appropriately.
But in the Back-end you’ll also see “Pages”. This is a Post Type -which is simply an organisational structure to separate different pots of data. So by default, WordPress expects Posts to be blog posts, and Pages are for non-timestamped data, like an About page.
This then leads onto the notion of hierarchical and non hierarchical data.
Hierarchical Structure
Posts that can be given a Parent/Child relationship, which works much like a traditional folder structure. If your pool of data needs section-like organisation, this is a great way to do it.
Non-Hierarchical Structure
Unlike the folder-style structure of Hierarchical Post Structures, this is a flat structure, where there is no notion of parent/children. Flat structures are great for huge amounts of data.
Example: Pages (Post Type) and Categories (Taxonomy) are Hierarchical, but Posts (Post Type) and Tags (Taxonomy) are Non-Hierarchical.