What is the difference between Sink and Stream in Flutter?

Sink and Stream both are parts of the StreamController. You add a data to the StreamController using Sink which can be listened via the Stream.

Example:

final _user = StreamController<User>();
Sink get updateUser => _user.sink;
Stream<User> get user => _user.stream;

Usage:

updateUser.add(yourUserObject); // This will add data to the stream.

Whenever a data is added to the stream via sink, it will be emitted which can be listened using the listen method.

user.listen((user) => print(user)); 

You can perform a various number of actions before the stream is emitted. transform method is an example which can be used to transform the input data before it gets emitted.

A StreamSink is a StreamConsumer, which means it can take several streams (added by addStream) and processes the events these streams emit.

If it is the StreamSink of a StreamController then all events from the added streams are emitted by the stream created by the StreamController.

This way you can pipe (forward) one or more streams into another one.