relation between "virbr0" / "virbr0-nic" network interfaces

[vrms]

I am trying to understand KVM networking a little better and have noted
that the "virbr0" network interface (the default KVM bridge) comes with
another device named "virbr0-nic".
The same kind of pair comes with each new bridge you may create via the
"virtual machine manager" (and assumingly other KVM tools alike)
Like "virbr1" comes in a pair with "virbr1-nic" (the *-nic interface is
being created automatically (and I assume will disapear automatically if
you remove the parent interface)

Can anybody kindly explain how these pairs are related to each other
and/or work together?

thanks ... Gunnar