Writing A Business Plan:

A business plan is a document which gives a detailed explanation of the mission statement, operation, market, the profit etc of a company (or business to generalize it). It is a document which acts as an open window to your business through which others (or even you) can get a better insight of the company.

A business plans has following parts:

An executive summary:
A short half to one page long description of key business points like purpose of the business, methods of operation, finances, management and a closing statement.
Describe the business:
Start with a short description on the current industry like a background search on the current and estimated future players. Give an overview of the product or service, nature of the business like is it a startup or acquired, manufacturing, retail etc. and the most important questions like how will you price it and how would it help make profit and do you place your different from that of the already existing ones?
Describe the market:
Describe size of the market, the current trends, estimation of sales and the method of sales.
Also mention about the distribution methods and the projection policies which includes your analysis on market trends governing your product popularity.
Mention about the type and size of competitors:
An analysis on the strength and weaknesses of the competitor is to be mentioned and showed how has it been used in the product developmental stages. Also define your distinct competitive advantage over your competitors while defining your strategies to use it in attracting market and sustaining.
Design and developmental plans:
It covers the goals, procedures and cost involved in terms of money and muscle (employees) for product development, market and organizational development.
They also can include the Operation, sales, marketing and advertizing plans.
Management and organizational structure:
This section becomes crucial as several businesses fail due to lack of proper management. Describe the executive management structure and organizational hierarchy properly.
The financial component:
Includes an assessment of risks and details of cash flow statement, income sheet, balance sheet, projected revenue, exit strategy etc.
Estimated growth rate:
It can be included at the business description level, but if you have enough data for a complete analysis of the growth rate, this can be a separate section.

Sample biz plans:
Entrepreneur . com
Businessplan . org
Post from Yale