Different sales and information channels through which you are reaching your payers, admirers, network and society.
Traditionally those are shop, resellers shop, online shop where the goods are sold or service provided. For information traditional channels are e-mail, TV, press, web-page. Some businesses use agents, door-to-door marketing, sales pyramids, distributors, tele and internet marketing.
Some businesses use “more is better” approach, some use just exclusive one or two sales channels. It depends very much on the philosophy of the business, complexity of the selling item, its price, status, positioning and many other factors.
The difference between Channel and Interface can be easily explained by allegory of the house. If you as a business are inside the house (and the house itself as a part of business) – windows, doors, underground tunnel, chimney, phone line and internet connection is how you get yourself out. Interface is this house, its windows, doors, external entrance of the underground tunnel and the chimney from the outside – how people around see your business.