The Apple Watch can also display iPhone notifications on your wrist. This includes other apps like Facebook, Gmail and pretty much anything that pushes a notification to your iPhone. You can control the notifications so you aren't overwhelmed by incoming Candy Crush invites, Facebook Messenger or other frequent notifications.
The Apple Watch app lets you configure notifications so that you are not constantly feeling a vibration on your wrist that can annoy you. We keep our notifications to a minimum.