The Common App (used for most private colleges)
The Common App is a free online college application delivery system that is used by over 700 colleges and universities. Although some out-of-state public universities use it, you'll find that most of your private college applications will be prepared and delivered through Common App. Know that while most of your private college applications will be submitted through this site, not every single college uses it. For example, CalArts and other art colleges tend to have their own online applications. Check the admissions page of your colleges to receive specific application instructions.
Common App Features -- There is one main application and essay you'll complete that will be received by every subscribing college you choose. However, oftentimes any given college also will require you to submit its own special application supplement that you'll also find on Common App, which will usually consist of some additional questions, short-answer essay questions, and/or an additional longer essay. As of 2017 the application will save your searches and account information, so go ahead, set up your own username and password, and get busy.
You will not be using the Common App for your CSU or UC applications. See their links on this page.
University of California Campuses-General Information
This is a good website for picking up general information about the nine UC campuses. However, in order to apply to one or more UCs during the strict November 1st - 30th application window, go to the admissions page of any UC campus website, and hit the "Apply Now" prompt. It usually doesn't appear online until around October 1st. However, you won't be able to actually file it until November 1st. One applies only once to multiple UCs by checking off each desired campus from a pulldown list on the online application. Each chosen campus receives the same application.