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.