Macvtap devices?

[X Tec]

I have used the -netdev tap,[...] options before, though almost exclusively for 
bridged networking.

I understand this setup uses tap (or tun/tap?) virtual devices for the virtual 
machines.

But, what are "macvtap" devices really?
I saw the term while reading some libvirt docs for comparison purposes; they 
particularly seem to favor these devices...
But even after internet searching I was not able to understand them.

So, if someone could help,
What are they, or what's their difference with "normal" tap devices commonly 
used in QEMU?
Are macvtap devices supported in QEMU? How can one use them?

Thanks beforehand.