Curriculum is the defined statement that maps out the interrelated elements of a group of related courses of study. A curriculum is "prescriptive" and sets out the educational tasks and experiences of the grades children pass through. A curriculum details the classes and class work students will engage in and may be determined by an external authority like the Department of Education.
Elements central to the classroom and related to the curriculum are: the skills to be learned by the students, the activities and strategies to be employed in the learning of the skills, the materials that will be used to learn the skills, and the assessments that will be used to measure student learning to determine level of mastery. The importance of a defined, documented curriculum is that it provides a balanced road map for the teacher and the students in a course. If a teacher thinks about teaching a class without all four of the above elements, then student learning suffers. For example, in a writing class, the teacher should have a list of specific writing skills he wants the students to learn and he should create small lessons to teach and allow for student practice. He should consider which books, programs, or resources he will use to aid in student learning, and he should know what the final product will ideally look like. For a teacher to just say, "I want students to write a personal narrative" isn't enough. He must ask himself what smaller skills make up a successful narrative. He should look for appropriate samples for the students to review. He should create specific practice writing activities to address specific skills, and finally he should put those skills together in an assignment that will measure the student's ability to use those skills to write a successful essay. The rubric or scoring of that essay should be an evaluation of the skills taught.
I personally think that many times teachers get too caught up in the "what I want to teach" that they forget the reason for teaching is skills. They say to themselves, "I really want to teach The Great Gatsby," but they don't ask themselves why they want to teach it or what the students will learn by having read it. They should instead ask themselves what reading or writing skills they can teach using the novel as the material. The title of a novel, a time period in a history class, or topic in any class may be essential to the content of the class, but these are only individual aspects of fulfilling an entire curriculum.