Written by Dorothy Tu
Lectures are taught by your professor - you go in, sit with EVERYONE enrolled in the course, and listen to your professor teach! Discussions are smaller, usually with 15-30 people, and are taught by GSIs (graduate student instructors). These discussions are where you will be graded, assigned homework (and turn in), be taught supplementary material to lecture, etc. GSIs will also usually take attendance, so while you may skip lecture because the room is too damn filled, you should not skip discussion sections because you'll lose points and the GSI will notice.
It is also imperative to attend lecture and discussion during the first week or two to ensure your spot in the class. If the class is too filled, they will drop those who don't attend to let those on the waitlist in. If you are waitlisted, you would attend both lecture and discussion to snag a spot as soon as they open up. You also might talk to the professor to discuss how enrollment will work for those who are waitlisted (they might have you sign a sheet).