Business Development or "BD", is often being used inter-changeably with sales. From what I have observed in Asia, BD is either seen as part of the M&A function with financial components, or it is simply a sales function. In fact, it is an overlay - a layer interacting with all activities right after strategising, marketing and sales.
In B2B business development, it is best understood as a value chain of activities leading up to sales. The BD team works on unlocking each stage of the value chain so that sales is made possible at the end of the chain. It is the operationalisation of the business strategy you have putting together. It is "doing business" in essence.
Strategyzer's Business Model Canvas provides a visual and organised way of understanding this value chain.
The Business Model Canvas includes components like partnership, production and operation, financing, customers, suppliers ...etc. It is not to say the the BD team has to be working on each and every function, however it is its prerogative to negotiate and align all these functions, so that business is made possible if he wants the stars to be aligned for sales to take place.
In a huge MNC, this value chain is taken care of by multiple individuals and functions. However, the role of BD team remains eminent, like a project manager that ensuring there is no "blood clots" at any stage.
In a small business, this is likely to be handled by the boss and/or senior personnels.
Then what about sales?
Sales is function at the end of the value chain. The BD team opens up every door; the Marketing team prims up the market readiness, then the Sales team converts and close with contract.