Sisweb is the one stop place for registering for classes (with the help of the Class Schedule and Registration Guide and the General Catalog) and for your financial aid. It interfaces with the BANNER Student Information System.
It replaced the old RSVP phone registering system.
Ninja Courses lets you create hypothetical UC Davis schedules to decide what you're going to register for.
If you're a UCD student, odds are that SISWEB will piss you off at one point.