I suggest you to go through two excelent tutorials which have potential to clear up the mist in your head (hope you don’t mind the pun):
Different bridge personalities: http://forum.mikrotik.com/t/routeros-bridge-mysteries-explained/147832/1
VLAN setup on bridge: http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1