NotifyPropertyChanged

Start new WPF application, add new model (class), like:

public class MyNotifyModel: INotifyPropertyChanged
{
	private string myName;

	public string MyName
	{
		get { return myName; }

		set
		{
			myName = value;
			OnPropertyChanged("test");
		}
	}

	protected void OnPropertyChanged(string name)
	{
		PropertyChangedEventHandler handler = PropertyChanged;
		if (handler != null)
		{
			handler(this, new PropertyChangedEventArgs(name));
			if (name == "test")
			{
				MessageBox.Show("tttets");
			}
		}
	}

	public event PropertyChangedEventHandler PropertyChanged;
}

Where XAML is like: 

<Window x:Class="IMyNotify.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:system="clr-namespace:System;assembly=mscorlib"
        xmlns:iMyNotify="clr-namespace:IMyNotify"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <iMyNotify:MyNotifyModel x:Key="IMyNotify" />
    </Window.Resources>
    <Grid DataContext="{StaticResource IMyNotify}">
        <TextBox HorizontalAlignment="Left" Height="23" Margin="159,67,0,0" TextWrapping="Wrap" Text="{Binding MyName, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Top" Width="120"/>

    </Grid>
</Window>

Important line to notice is:

UpdateSourceTrigger=PropertyChanged - that means that onPropertyChanged event will be raised when someone enters something in textbox field

Example you can download from here.

Now, if we start from scratch, your model should look like: 

public class MyNotifyModel
{
	public string MyName { get; set; }
}

Now, lets add INotifyPropertyChanged, like: 

public class MyNotifyModel: INotifyPropertyChanged
{
	public string MyName { get; set; }
}

Now we will have to implement members, so now model will look like:

public class MyNotifyModel:INotifyPropertyChanged
{
	public string MyName { get; set; }
	public event PropertyChangedEventHandler PropertyChanged;
}

After that you have to introduce new variable, in my case that was myName, set getters and setters as I already describe it on the beggining, and implement OnPropertyChanged event.

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2014-06-13 Update Better version of notify property changed explanation you can see here.