This is a link that allows others to view your item inventory on Steam. You will need to provide this to sites to trade items in or out.
Yes! This only allows others to view your inventory and sent you trade offers. In order to send or receive an item, you will need to manually confirm the transaction within Steam.
The easiest way is to follow the steps above. You just need to login to Steam through the link above and scroll down to the Third-Party Sites section.