Selling is the process of taking a prospect and converting them to a customer.
You will find other definitions, many of which are over-complicating or inaccurate. That's the most succinctly I believe it can put.
Although processes will be different depending on whether you are selling potatoes from a market stall or contracting on high end networking agreements, to learn about being good at selling is mastering the art of success.
For the market trader, shouting out the price and availablility of his potatoes is his marketing. When the customer comes to the stall and he happily persuades the customer to buy a few extra spuds and a bunch of bananas, that's selling.
Contractual sales are evidently more complex and might include several demonstrations, legal negotiations and a protracted sales process, so I have dedicated a page to the subject. Good sales people here will be more methodical but still need a good understanding of salesmanship.