A syllabus is a document that summarizes and outlines the most important information about the course. It includes your name and the title of the class, its learning outcomes, the course content, and a schedule of activities. It also includes detailed information about attendance policies, grading standards, and supply and/or reading lists.
The exact contents of a syllabus vary widely from teacher to teacher. Some faculty hand out syllabi of many pages, some are only one or two pages long. Regardless of length, a syllabus sets the contract between teacher and student for the duration of the course and, as such, should be adhered to.
Any changes you make to the syllabus in the middle of the quarter should be given to students in writing and announced at least twice verbally during class. This is very important in case a student should later have a grievance.