A business plan is a written description of your business's future, a document that tells what you plan to do and how you plan to do it. If you jot down a paragraph on the back of an envelope describing your business strategy, you've written a plan, or at least the germ of a plan.
Business plans are inherently strategic. You start here, today, with certain resources and abilities. You want to get to a there, a point in the future (usually three to five years out) at which time your business will have a different set of resources and abilities as well as greater profitability and increased assets. Your plan shows how you will get from here to there.