The singular function of business development is to figure out how to get your product or service into the hands of clients or customers, ultimately, to increase the value of your business by gaining market share or directly generating revenue.
It's easier to think of business development as a role within an organization in the context of that function. A business development role can include a wide range of responsibilities depending on the needs and stage of a given company.
One of the most exciting and challenging roles for a BD person is to help a start up find product-market fit. That might mean one-to-many 'selling' to drive user adoption of the service/product or one-to-one 'selling' to advertisers or licensees. In either case, what makes this challenging there is little or no process or formula for success. For example in the later case, it's on the early stage BD person to discover the practical need and value of the product or service in the marketplace through trial and error with partners.
At more established companies that have reached product market fit, the spectrum of BD roles can vary. It can range from a sales person with a target/quota that has a BD title because the sale is considered 'highly consultative' to a more strategic BD person who is responsible for taking a more broad view of the business and generating new businesses/revenue streams based on the current businesses assets.