The features are basically what you'd expect. The block feature will let you block a sender and then emails from a blocked sender will go to your spam folder automatically. If you block someone accidentally or have a change of heart, you can always remove them from the block list via the settings. Google says the block feature will go live on the web today, but roll out over the course of a week on Android.
Unsubscribe is, again, just that. If you get newsletters or emails that you were subscribed to without knowing when or how it happened, you will soon be able to clean those up much easier. The unsubscribe button has been on the web for a while, and clicking it will unsubscribe you from those pesky emails without having to go through the annoying, "Can you tell us why you're unsubscribing?" survey. Google doesn't expressly say it, but we assume this button will also roll out over the next week.
source: Gmail Blog