A powerful marketing plan is essential for the success of any company or brand. Without it, no marketing strategy is complete—or viable.
And what is a marketing plan? It's "a comprehensive document or blueprint that outlines the advertising and marketing efforts for the coming year. It describes business activities involved in accomplishing specific marketing objectives within a set time frame.... The marketing plan shows the step or actions that will be utilized in order to achieve the plan goals." (Wikipedia)
Your marketing plan might be a highly formal, detailed document, or it can be informal and less detailed to allow for flexibility—or somewhere in between. In any case, most good marketing plans have various components in common.