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

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