When I ask entrepreneurs and small business owners, it is not unusual for not having any marketing plan. I once read that Sir Richard Branson never had any marketing plan when he started the 1st Virgin business many years ago. In fact in one study of entrepreneurship (I couldn't recall the name of the study) was that as much as 50% of entrepreneurs and business owners don't have a business plan and marketing plan when they first started the business.
Marketing Plan: In this you perform, precise activities which helps in achieving the goals and objectives you are targeting at. Here, you plan how you will promote the product/service? What are the specific areas of promotion? What would be the budget of this plan execution? Etc. Plus, you also estimate the ROI of this marketing strategy.