An open listing is the type most often indicated by a "For Sale by Owner" (FSBO) sign. The owner retains the right to sell the property themselves and pay no commissions to anyone. This owner/seller is many times willing to pay a commission to a real estate agent that brings a buyer. They might indicate that in an ad or on a sign with "brokers welcome" or "brokers protected". That does not however constitute a listing agreement, and a wise agent will execute some type of written commission agreement before bringing a buyer to the property.
Exclusive Agency Listing:
This listing type is also a for sale by owner with the owner retaining the right to sell it themselves without paying anyone a commission. However, in this type they will also execute a listing agreement with one brokerage and pay them a commission if they are the procuring cause
of the sale. No other brokers would be invited or paid.
Exclusive Right to Sell Listing:
This type of listing provides the greatest commission protection for the listing real estate broker. The property seller agrees to pay the listing broker a commission no matter who brings the buyer, even the owner themselves meeting them on the street.
This type of agreement also benefits the sellers, as the brokers in the other two types would not want to expend great efforts or expense in marketing the property due to the commission risk. In this type, the broker is protected and more willing to invest in the marketing.
Before You Take Your Buyer to View a Listing, Know the Type:
In aggressively representing their buyers, particularly in markets with low inventory, real estate agents might be out looking for any property available. It's important to their income that they know the listing type before the buyer is taken to view it. If it's one that is not Exclusive Right to Sell, then the agent needs to take steps to protect their right to a commission up front.
The many MLS, Multiple Listing Services, generally are made up only of Exclusive Right to Sell Listings, as the listing broker is offering a portion of the commission to any member broker or agent that brings a buyer. In order to do that safely, the listing broker would need to be assured of the commission from which they would pay the buyer side share.