Although a virtual switch can connect the NICs of a number of virtual machines together, the switch and the NICs of VMs remain invisible to the host OS (from the networking perspective).
To this end, the host OS can create a TAP device. a TAP device is a virtual network port, but available to the host OS (as opposed to the VMs). A TAP device is essentially a virtual ethernet port. What you can do with a physical ethernet port can also be done to a TAP device.
VDE has a special feature to connect to a TAP device. In other words, a TAP device of the host OS can connect to a port of the VDE virtual switch. This allows a host OS have full access to all the VMs connected to the virtual switch by means of normal networking.