As the web becomes more mobile, Facebook is targeting a large audience: 2500 different mobile phones. The App is designed to be data efficient and to provide the users with a smooth and light interface that includes News Feed, inbox messages and the ability to upload photos from your phone.
It is not very different from a native app designed for Android, iPhone or Blackberry but lighter. You can get the app in m.facebook.com from your mobile browser and Facebook will pay you upto 90 days of data usage (only app of course) to get more people to try it out.
Here is a preview of what the app looks like.