Well, they help you find a house to buy (or a rental these days). The realtor also ensures that the contract is proper/legal, that there are no surprises (house has issues, damage, etc), and that you don't get ripped off, and that you find a house that is appropriate for *you* (according to your needs and wishes). They also negotiate on your behalf if you are buying/selling (not sure how negotiable rent prices would be, though...).
Cost for purchases is a percentage of the total sale price of a home. One does not pay to merely be shown homes. However, if you end up buying a house, it's the seller that pays the realtor, not the buyer.
I am not sure who pays/how much is paid for finding someone a rental. Contacting a realtor, however is always free - so these questions could also be answered by the realtor himself.
Mind you, I don't mean to advertise. Perhaps you don't need / want a realtor! I just mentioned it because I know for SURE this guy would be very helpful and trustworthy, and if you don't know anybody, it can be difficult to pick someone. If you are interested, I can send you the contact details.