By default the forums will send you a weekly summary of the most recent posts. You can also follow certain categories so that you will receive an email when someone adds a new topic.
To follow a category, go to the categories page and then select the category you want to follow. Once you are in the category, you will see a menu in the top right with a circle in it:
If you click on this menu you can choose from several levels of notifications for the category:
