A business plan is a written document that describes the business, outlines business goals, and details the route that will be taken to achieve those goals. A business plan is not a static document that you write once, and writing a business plan doesn't automatically guarantee success. On the other hand, a business plan will allow you to set targets for your business and to communicate your strategy and vision for your team. It also gives you a framework to evaluate your strategy and your financials and determine whether you need to make any changes. Writing a business plan also gives you a document to show banks and other potential sources of financing that your business is viable, and a solid plan can also help you negotiate favorable rates from banks and investors. The key components of a business plan include: a description of the company's organization, management, and products or services; financial projections; funding requirement; analyses of the industry, market, and competitors; and sales and marketing strategies.
Every business should have a business plan that projects three to five years into the future. Think of the business plan as both an operational roadmap and a "living document" that evolves as the business develops.
The video below was created by Bplans.com, and explains this in greater detail.