Some distribution channels charge a fee, or sales commission, which is paid by you. This is the distribution channel's fee for bringing the customer to book through their website and/or for listing your business on their website or advertising your business online.
A distribution commission is generally a percentage calculated based on the total price paid by the consumer.  Booking.com, Airbnb and Expedia use this model. Alternatively, some channels charge a fixed listing fee or an ‘advertising fee’ based on pushing customers to you at a cost per click. Some do both. Google and Tripadvisor are examples.
Some distributors also charge the customer a fee, for example Airbnb, Homeaway.