I am working on a Wordpress site that users can login to and make posts. I want to add some functionality to their posts. Basically, I want to provide the ability for the user t
You're on the right track.
Take a look at WordPress Hooks. You're interested in the edit_post hook.
Within a custom callback you can parse the content and look for your category hashes. Then use native WordPress functions to query an existing category or create a new one. Most of the complex logic will be in the latter part. The rest should be string manipulation.
This doesn't necessarily have to be a plugin. This could live in your functions.php file of your theme.
Remember, the codex is your friend.