In your Diary you keep track of your most memorable moments and milestones in the game.
You can look through your friends' Diaries when you visit them, and they can see yours.

The Diary consists of 3 parts: Journal, Balance and Wonders.
The Journal contains chapters with different interesting tasks. After completing each chapter you get prizes.
The Balance shows the stats of your island's infrastructure and provides you with advice on how to make the resort perfect and to get bonuses for that.
On the Wonders tab you see what unique animals and objects you can have. You can acquire what you want, and it will appear on your island.