What is difference between " * " and "Auto" in Silverlight Grid Layout Definitions

trying to understand the following:

<Grid Name="Root">
    <Grid.RowDefinitions>
        <RowDefinition Height="*" />
        <RowDefinition Height="*" />
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="*" />
        <ColumnDefinition Width="Auto" />
    </Grid.ColumnDefinitions>
</Grid>

Can anyone help me in explaining the difference between * and Auto in the above snippet?

thanks