Bookmarking is bookmarking in your web browser. You aren’t alerted when there’s an update, but you can come back to the topic later. Subscribing, however, will notify you when there is an update to the topic or forum on the site via your preferred method or methods.
To subscribe to a specific forum, click the "Subscribe to the forum” link upon entering the forum, you can also get an rss feed. To subscribe to a topic, reply to the topic with the subscribe checkbox checked or click the "Enable e-mail notifications about answers” link within the topic itself.