What is a Business Plan?

A business plan is any plan that enables a business to look ahead, allocate resources, focus on key points, and prepare for problems and opportunities. Businesses need business plan to optimize growth and development according to plans and priorities.
This kind of plans usually is written for different purposes. It's important to make your plan match its business purpose. For instance, if you want to submit it to investors or banks you should provide more information about what you are and where your position is in financial terms, but if it has internal purpose you can avoid some detail information about situation of your firm or its market
There is no standard outline for business plans but always take your audience standards into account. Remember that usually cash flow and implementation details are most important parts of every business plan independent from its purpose. But market research depends on objectives. For instance, detail market research is necessary if there is an opportunity to improve the company by learning more about market. (For small group of people that are near to company too much detail isn't necessary)
Reference: HURDLE, a step-by-step guide to creating a thorough, concrete and concise business plan, written by Tim Berry, 2006.