Mostly a camera which is near the display on the front side. Thus allowing to preview the image to be taken.
Wikipedia:
A front-facing camera is a feature of cameras, mobile phones and similar mobile devices that allows taking a self-portrait photograph or video while looking at the display of the device, usually showing a live preview of the image.
A camera on the front of the phone, facing the user. This enables two-way video calls (such as Google Hangouts, Apple FaceTime, or Skype), and is also useful for capturing self-portraits ("selfies").
