A thread is a discussion within one of the forums. Such as this one that you started. ;-)
A post is an individual comment within a thread.
There are a couple of ways to find all your forum posts.
1. If you click on your username that is in one of your posts, a drop-down box appears and one of the selections is 'Forum Posts'. When you click on that, it will bring up a list of your posts in chronological order (the last post you made will be at the top of the list).
2. If you go to your profile here on Chatter, you'll see the total number of posts that you have made. That number is a live link to a list of the posts themselves.
If you want to search the forums for a topic that has been discussed before, use the Search Function that is provided with this website. Here is a link to a thread that describes how to use the Search Function:
Search Function Hints and TipsCheers, Dean