Linux-kvm bridges
TAPs are userspace virtual interfaces (as opposed to TUNs which are at the ethernet frame level. This linux-kvm tip on bridged networking and performance explains some of the basics. There's a lot more at the linux-kvm site to digest.
This impressively detailed post configures a bridge manually
This guy claims to have figured it out
The Arch wiki has a section on qemu and bridging modes and tap devices which claims that the breakdown is:
User-mode networking By default, without any -netdev arguments, QEMU will use user-mode networking with a built-in DHCP server. Your virtual machines will be assigned an IP address when they run their DHCP client, and they will be able to access the physical host's network through IP masquerading done by QEMU. This only works with the TCP and UDP protocols, so ICMP, including ping, will not work. This default configuration allows your virtual machines to easily access the Internet, provided that the host is connected to it, but the virtual machines will not be directly visible on the external network, nor will virtual machines be able to talk to each other if you start up more than one concurrently. Tap networking with QEMU QEMU can use tap networking for a virtual machine so that packets sent to the tap interface will be sent to the virtual machine and appear as coming from a network interface (usually an Ethernet interface) in the virtual machine. Conversely, everything that the virtual machine sends through its network interface will appear on the tap interface. Tap devices are supported by the Linux bridge drivers, so it is possible to bridge together tap devices with each other and possibly with other host interfaces such as eth0. This is desirable if you want your virtual machines to be able to talk to each other, or if you want other machines on your LAN to be able to talk to the virtual machines. Host-only networking If the bridge is given an IP address and traffic destined for it is allowed, but no real interface (e.g. eth0) is connected to the bridge, then the virtual machines will be able to talk to each other and the host system. However, they will not be able to talk to anything on the external network, provided that you do not set up IP masquerading on the physical host. This configuration is called host-only networking by other virtualization software such as VirtualBox. Internal networking If you do not give the bridge an IP address and add an iptables rule to drop all traffic to the bridge in the INPUT chain, then the virtual machines will be able to talk to each other, but not to the physical host or to the outside network. This configuration is called internal networking by other virtualization software such as VirtualBox. You will need to either assign static IP addresses to the virtual machines or run a DHCP server on one of them. Bridged networking using qemu-bridge-helper Note: This method is available since QEMU 1.1, see http://wiki.qemu.org/Features/HelperNetworking. This method does not require a start-up script and readily accommodates multiple taps and multiple bridges. It uses /usr/lib/qemu/qemu-bridge-helper binary, which allows creating tap devices on an existing bridge.
It also goes into the alternative methods using VDE.