openvpn, tap vs tun, differences

While creating an OpenVPN server you should think what you are going to use it for.

Long story short

Use tap for a layer 2 vpn, tun for a layer 3.

tap

Tap is a virtual eth interface and, you will be able to put the interface tap0 into a bridge:

apt-get install bridge-utils
brctl addbr br0
brctl addif br0 tap0 eth0

tun

Tun is for a direct IP connection between two points.

Link to official OpenVPN page.