A list of overarching goals for a company, along with an outline of how the company plans to accomplish those goals.
A business plan, as defined by Entrepeneur.com is:
A written document describing the nature of the business, the sales and marketing strategy, and the financial background, and containing a projected profit and loss statement
It is useful to have a business plan, as it sets down what you'd like your business to accomplish.
Writing a Business Plan
There are many good resources to help one write their own business plan. Of note: