Business Development is a very ambiguous job title and encompasses a lot. However, simply put, as a business developer you are responsible for promoting the product, identifying any improvements the product may need and bringing it to new markets. As the person responsible for bettering the business, your responsibilities will include contacting potential clients and arranging meetings, researching the product and potential markets for new opportunities and attending conferences, meetings and events. In principle you will be the face of the business. The business development team also works with the product management team to identify the products competitive positioning and give suggestions on how to improve the product.
Business Development vs Sales
Business development is the process of matching the product you are promoting with new potential markets to expand into. You will guide potential new clients through the entire sales pipeline.
Sales is purely focused on initiating new revenue streams. The business development team will be pointing the sales team into the new markets they have identified. In sales you will be pitching clients and winning business.