Categories organize and contain the topics. The "Main Categories" link near the top left will take you to the main categories. Clicking on a category will display the sub categories. As you continue to click the sub categories you will eventually reach the topics that contain the messages.
Topics contain a group of messages relating to the topic's name. Click on a topic to see a list of the messages concerning that topic.
Groups of messages are found inside topics. Messages submitted under a topic usually (but not always) are concerning the topic's title.
You can enter your own message by clicking the "Add Reply" button that can be found above and below the list of messages.