If you would like to receive an email notification when there is an update in board or topic, you can subscribe to that board or topic. If you are already subscribed, then you have the option to unsubscribe so that you no longer receive the email notifications. You can control when you receive these notifications under My Settings > Subscriptions & Notifications > Notification Settings.
The RSS Feed is similar in that it will alert you about updates. However, you will be notified through an RSS Feed Reader rather than an email notification. For more information about how RSS works, refer to this article.