Well, it looks like a normal messaging app but it has some tricks. For example, you can change the size of the message. When you write something, you can hold the sent button. If you swipe it down, the message will be smaller while if you swipe it up, the message will be bigger.
The best feature is the smart reply. For example, if your friend asks, “do you want to go to the dinner?”, it will give you few options such as “Yes” or “no”. The smart reply also works with pictures. For instance, if your friend sends you a dog picture, the app will give options like “Nice Dog”.
You can also use Google Assistant within the app. You can find a restaurant and even make a reservation within the app.
There is even a emoji game in the app powered by Google Assistant. In this game, Google will show emojis, and you will need to name the movie.
There is an incognito mode in Allo app where the messages are encrypted and the messages will be only showed up on your device.