Posts are visible to anyone, even non-PNut users. They're tacked together into lists of posts called "streams". The Global stream has all posts. If a post is directed at you, it will be included in your Mentions stream. There are lots of streams to view posts in.
Posts are limited to 256 characters in length. They can be edited, but only once, within 5 minutes of creation. And when you edit it, you can change any text, but you have to include all the same [mentions], [[links]], [[tags]], and other fancy things like [[spoilers]], [[embeds]], and [[polls]].