If an online business, you need to go into detail how you will attract customers to your website. General statements like "I will use Face Book ads and email marketing" will contribute almost nothing to helping your cause unless you have detailed statistical analysis of tests you have conducted or of another similar business you have been associated with. If you do not have any data upon which you reference your estimates, it could show lack of proper thought to the remainder of your business plan.
Financial Plan _ The financial plan consists of four sections: Financial Worksheets, Cash Flow Projections, Financial Statements, and Additional Financial Information. All of these components will tell the story of how you plan to start or grow your business from a financial perspective. It is vital that you explain the assumptions under which you have based your projections, for example, "We assume that there are no unforeseen changes in economic policy to make our products and service immediately obsolete." or "We assume interest rates will stay the same over the next three years." (both quotes from Bplans.com sample business plans)
Projected Financial Statements: These statements are usually helpful, but not necessary. You will develop and describe your strategies for the business throughout your Business Plan. In the financial section, you will need to estimate the financial impact of those strategies by developing projected Income Statements, Balance Sheets, and Cash Flow Statements.
So, although a detailed business plan may not be required for an online business, I am going to include it here so you can at least look at and consider each section and determine yourself if it applies to your business.