Ghost has a flexible organisational taxonomy called tags, which can be assigned from post settings menu within the Ghost editor.
Tagging
By tagging posts with one or more keyword, you can organise articles into related content.
For example you may tag some content with News and other content with Podcast, which would create two distinct categories of content listed on /tag/news/ and /tag/weather/, respectively.
If you tag a post with both News and Weather - then it appears in both sections. Tag archives are like dedicated home-pages for each category of content that you have. They have their own pages, their own RSS feeds, and can support their own cover images and meta data.
Primary Tag
Tags can drag and dropped into a specific order from the post settings menu. The first tag in the list is always given the most importance, and some themes will only display the primary tag (the first tag in the list) by default.
News, Technology, Startup
Private Tags
In Ghost, hashtags are private tags and can be used for special styling.
For example, if you sometimes publish posts with video content - you might want your theme to adapt and get rid of the sidebar for these posts, to give more space for an embedded video to fill the screen.
News, #video
Here, the theme would assign the post publicly displayed tag of News - but it would also keep a private record of the post being tagged with #video.
You can find documentation for theme development techniques like this and many more over on Ghost's extensive theme documentation.
Posts & pages
There are two types of content in Ghost than can be organised with tags: posts and pages. Posts are the primary content type and posts will belong to an index page or collection of posts. Pages are generally used for static one-off content, and are excluded from all feeds. Find out more about posts and pages in the core concepts guide.