A business plan is a written document that describes all internal and external elements and strategies for starting a new business venture. It integrates functional plans such as marketing, finance, manufacturing, and human resources. Developing a business plan is important as it helps determine market viability, provides guidance for planning activities, and serves as a tool for obtaining financing. Once the business is started, the entrepreneur should regularly monitor key metrics like profit/loss, cash flow, inventory, and sales to measure progress against the original plan.