UPDATE:
By replying to a topic you are automatically following it. However, if you'd like to follow a topic without replying to it click the "Follow this discussion" link for a "Talk" post, the "I have this problem too" button for a "Problem" post, the "I have this question too" button for a "Question" post, or the "I like this idea too" button on an "Idea" post.
Following a topic means you'll receive email notifications when an update is made to the topic and the topic will appear in your Dashboard.
If you want to "un-follow" a topic click the "stop following it" link.