A business plan is simply a plan of what your goals are for the business, and how you plan to go about accomplishing them. Some people refer to it as a vision for your business. I like to refer to it as a blue print or a road map to accomplishing your goals. Your plan should be spelled out in clear and defining terms and be kept simple. It should be a written document and used as a tool in managing the business.
When you, and your partners if any, have all the core skills and industry knowledge you need to start right away without seeking experts, napkin notes may be enough to get going. Let's say you are already an expert in technology and social media. Then you, and your team, probably don't need a detailed plan to start developing a new app. You will draw on your knowledge and experience, and you understand that you might need to go back and do some more detailed and formal planning later. Certainly when you reach the point where you are looking for investors or lenders, you will move beyond those first casual notes. Until then, drawing upon your expertise can allow you to quickly jump into the market and perhaps gain a competitive edge by using a minimalist plan.