rear derailleur adjustments

Hello, first of all, I hope this the right section for my post, if not, please to correct me; I am new to road bikes, - I have recently put together hybrid/road bike, but I can't figure out one thing - why is my rear dérailleur's small pulley so close to the cassette? I tried to adjust the dérailleur by manipulating with the screws, but nothing seems to help. when I pedal backwards the chain obviously isn't going smoothly and sometimes even `falls off`; this probably is a very simple question, sorry about that, but any help would be much appreciated