Content is what Piranha CMS is all about, and out of the box we provide four primary content types, Sites, Pages, Posts and Media.
Sites are the root node for your Pages and Posts and can be mapped against a specific hostname or subdomain. You can have several sites that will all have their own unique hierarchical content structure, but you need at least one site available.
You can read more about Sites here.
Pages are positoned in the hierarchical site structure and can be either regular pages, or archive pages. The main difference between them is that archives contains a unique collection of Posts, Categories & Tags.
You can read more about Pages here.
Posts are pieces of content that does not have a position in the sitemap other than the archive page they are positioned in. Posts can be organized with categories and tags and can be used for various purposes. In terms of content features they are very similar to pages.
You can read more about Posts here.
Media are assets uploaded into Piranha that are intended to be used in other content. Media assets are not published but rather relies on the published state of the content refering to it. The media library is also global for the entire application and assets can be used on all other content regardless of site or archive.
You can read more about Media here.
Pages, Posts and Sites are composed of Blocks, Regions and Fields. You can find more information about in their respective article.