Grids are used to organize other controls. Rows and columns can be defined so that other controls can locate themselves in the location they need to be. Example below shows a simple maxinpage.xml that uses grids and row controls a little bit

<phone:PhoneApplicationPage
    x:Class="BasicLayout01.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
    xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    SupportedOrientations="Portrait" Orientation="Portrait"
    shell:SystemTray.IsVisible="True">
 
    <Grid x:Name="LayoutRoot" Background="Transparent">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
     
        <StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,28">
            <TextBlock Text="MY APPLICATION" Style="{StaticResource PhoneTextNormalStyle}" Margin="12,0"/>
            <TextBlock Text="page name" Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
        </StackPanel>

        <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
            <Grid.RowDefinitions>
                <RowDefinition Height="1*"/> <!-- When used * means th of avalable space-->
                <RowDefinition Height="2*"/> <!-- Here it is 2/6 of available space -->
                <RowDefinition Height="3*"/>
            </Grid.RowDefinitions>
         
            <Rectangle Fill="Red" Grid.Row="0" />
            <Rectangle Fill="Blue" Grid.Row="1" />
            <Rectangle Fill="Green" Grid.Row="2" />
         
        </Grid>
 
    </Grid>

</phone:PhoneApplicationPage>


The output screen looks like that