What is a Lesson Plan?
There is some confusion about what a lesson plan* is and is not. A worksheet is not a lesson plan. A handout is not a lesson plan. A classroom game or activity is not a lesson plan. In fact, there is no need for a lesson plan to ever be seen, touched, considered or dreamed of by students, and nor does it even need to exist on paper or disk, though it usually does.
A lesson plan is a teacher's plan for teaching a lesson. It can exist in the teacher's mind, on the back of an envelope, or on one or more beautifully formatted sheets of A4 paper. Its purpose is to outline the "programme" for a single lesson. That's why it's called a lesson plan. It helps the teacher in both planning and executing the lesson. And it helps the students, unbeknownst to them, by ensuring that they receive an actual lesson with a beginning, a middle and an end, that aims to help them learn some specific thing that they didn't know at the beginning of the lesson (or practise and make progress in that specific thing).
To summarize, and in very basic terms: a lesson plan is the teacher's guide for running a particular lesson, and it includes the goal (what the students are supposed to learn), how the goal will be reached (the method, procedure) and a way of measuring how well the goal was reached (test, worksheet, homework etc).
The examples (right) show a fairly formalized handwritten lesson plan and a back-of-the-envelope lesson plan for the same lesson -- a 45-minute lesson on the Future Continuous.
The first lesson plan indicates the time in minutes to be spent on each activity and whether the (inter)action is teacher-student, student only or student-student (left-hand column) and materials to be prepared/used (right-hand column).
The back-of-the-envelope lesson plan is clearly by someone who has done this lesson many times before and has simply scribbled a brief reminder of main aspects of the lesson.
Both of these are examples only. There are many different ways of achieving the same objective.
* Note that we are talking here about lesson plans in the context of TEFL (teaching English as a foreign language). In other contexts, the term "lesson plan" may have a broader meaning, including something more akin to an entire curriculum.