Who can see what I post?

It depends where you put it:

• A public club — anyone on the internet, signed in or not, and search engines may index it.
• A private club, or a members-only thread — only people with access to it.
• A direct message — only you and the person you sent it to.

Your profile collects your posts, comments and likes, so activity in a public club is visible from your profile too. The safe assumption is that anything outside a direct message is public and permanent — people quote and screenshot, and deleting your copy does not delete theirs.