'Watching' is the new 'subscribing'

Exact same thing, different name. When you 'watch' something in the forums, that means you'll get notified of new posts/ threads.

Watching a thread

Click on the Watch Thread link at the top of each thread:

By default, you automatically 'watch' any thread you post in. To change that, and to get automatic email notifications of replies, you need to edit your preferences.

Watching a forum

Click on the Watch Forum link at the top of each forum:

You can choose whether to be notified of every new post, or just of new threads: