What's the difference if I let the listing agent represent me or I decide to hire a "selling agent"?
A listing agent is in contract with the seller to sell the home at the highest price possible. A selling agent represents the buyer and is responsible, amongst others things, to help the buyer purchase the home at fair price.
While it is legal to have a listing agent represent both parties, it is your right, as a buyer, to decide whether or not you want to use the listing agent or find a selling agent to represent your interests in the transaction.
If you are viewing a property during an open house, it is likely that a listing agent, or one of the agents assistants, will be showing the property. It's common for the agent showing the property to ask if you are "represented". If you are not represented, and desire to make an offer, you have the right to use the listing agent or quickly find a selling agent to negotiate and make the offer on your behalf.