A syllabus is your guide to a course and what will be expected of you in the course. Generally it will include course policies, rules and regulations, required texts, and a schedule of assignments. A syllabus can tell you nearly everything you need to know about how a course will be run and what will be expected of you. See "How Do I Read a Syllabus?" for more on that topic.
Where can I find syllabi?
Check the Syllabus archive. You can search by quarter and by department--if looking for previous syllabi, you can use ExploreCourses to see when the course was last taught.
ExploreCourses (not SimpleEnroll) has links to syllabi for many courses. Click on the blue Schedule link to see if there are Additional Resources for that course.
In addition, some courses have course websites that act as a syllabus, particularly the introductory courses in Math and CS. You can also contact the instructor and/or the SSO in an academic department/school to inquire about the availability of a syllabus, or ask to see a previous syllabus.
Heads-up! Syllabi are always subject to change, especially if a different instructor is teaching a course. However, even looking at last year's syllabus can give you valuable information as you are evaluating a course.