HI,What is the differenence between BDI & BVI,Why we need them ?what is the difference b/w them?can we get same feature with loopback interface also ?

Hi, i have multiple questions..For VPLS, we configure service instance, VFIm, Bridge domain..Can anyone tell me -how does the traffic use these three terms ...what is there in bridge -domain,,what is the use of VFI,WHAT is the use of Service instance...