Styles makes life easier ,you can reuse them within the entire Application.And it is very best practices styles can included in App.xaml page to available for entire application.
Implicit Style:
Here created style need not to have key identifier and you don't need to specify key identifier when you use this kind of style.
<Application.Resources>
<Style TargetType="TextBlock">
<Setter Property="FontSize" Value="8" />
</Style>
</Application.Resources>
Here you don't have to specify the style for the TextBlock , it will automatically pick the above style when you created TextBlock in your application.
Explicit Style:
Here created style must have key identifier and you must specify that key identifier when you use this kind of style.
For example ,
The developers can create a style for the textblock with a key identifier in the app.xaml file and then reuse them in different screens of the application.
<Application.Resources>
<Style x:Key="MyCustomStyle" TargetType="TextBlock">
<Setter Property="FontSize" Value="8" />
</Style>
</Application.Resources>
The above style is for the textblock is given a name with the key attribute "x:Key="MyCustomStyle" . To refer to this style for a textblock , use the sample code for the textblock as shown below.
<TextBlock Name="Txtblk" Style="{StaticResource MyCustomstyle}"/>