Difference between "tap" and "bridge" netdev options

[xtec]

Hello. New here.

I come from being using VirtualBox and QEMU/KVM through libvirt, and 
decided to start studying how to use pure QEMU/KVM.

So far I have managed to learn the basics, and a bit more I think.
I just reached the feared networking part. I'm aiming for bridged 
networking, so already created a network bridge in host and specified 
suitable qemu options.

However I'm still completely unable to understand one thing:
What's the difference between using "-netdev 
tap,helper=/path/to/qemu-bridge-helper" and "-netdev 
bridge,helper=/path/to/qemu-bridge-helper" ?
Or even furthermore, the differences between using "helper=" and 
"script=" ?

Thanks beforehand for your attention.