A post (or article) is one of the two types of content available by default in WordPress, the second being the page (see the page definition).
Posts are used for publications related to a given date, event or theme. They are mainly used in blogs and news sites.
The main characteristics of a post (or article) are:
- They have a publication date. In general, they are displayed from the most recent to the oldest.
- They have an author.
- They are linked to one or more categories, and possibly tags, in order to organize the content.
- They can be in different formats, depending on the theme: videos, quote, link, gallery, etc.
- They can have a featured image to illustrate them.
- They can have a comment thread to interact with visitors.
- They can appear in RSS news feeds.
- They can be shared via share buttons on different social networks.
- There is no limit on the number of posts/articles a site can contain.
- They should not be used in navigation menus.
To go further, check out this post which will explain in more detail the differences between posts and pages on WordPress.