Separators are the little tiddly-bits that, um, separate items in a menu or tool bar. 
The intention is to divide the items on the menu or toolbar into logical groups. Separators are controls, but they don't have any real behavior.
One of our current projects includes an exhaustive custom styling of a WPF application. Last week, I was working on the top level menu as well as context menus. I wanted all of my separators to look the same. So I ignorantly created a style, the same way that I would for any other control. It looked something like this:
<!-- Separator -->
<Style TargetType="Separator">
<Setter Property="Height" Value="1" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Separator}">
<Rectangle Height="{TemplateBinding Height}"
Fill="{StaticResource NormalBorderBrush}" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
In case you're not familiar with it, WPF allows to specify a default style for a control by setting the TargetType and omitting the x:Key.
But that doesn't work for separators.
The reason is that MenuItem and ToolBar both use separators, and thus creating global style for both wouldn't make sense. (At least, I'm guessing that's what the designers of WPF were thinking.) Instead, both of these MenuItem and ToolBar expose a static property, SeparatorStyleKey. This acts as the key for locating the corresponding separator style.
If you want to style the separators for all menu items, your need to set x:Key to this static property. The resulting style would look like this:
<!-- Separator -->
<Style x:Key="{x:Static MenuItem.SeparatorStyleKey}"
TargetType="{x:Type Separator}">
<Setter Property="Height" Value="1" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Separator}">
<Rectangle Height="{TemplateBinding Height}"
Fill="{StaticResource NormalBorderBrush}" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>