After schools receive your preliminary applications from centralized application services, you may receive secondary applications. Some schools do not request secondary applications, but base their decisions on the information contained in your preliminary application and letters of recommendation. Some schools send secondaries to all applicants, some to all in-state applicants. Some schools conduct a review of the preliminary applications to determine whether or not to send each applicant a secondary.
The secondary application usually asks focused questions, and may require more essays. You may end up repeating some of the information contained in your personal statement. This is OK (it means you chose to write about information they were interested in!). However, do not repeat your personal statement. The schools are looking for new essays and more depth.
In some cases, schools may request other supplementary materials such as letters of recommendation or resumes. Do not send any supplementary materials, including letters of recommendation, to the schools until you are instructed to do so. Follow all instructions carefully. Send all materials in as soon as possible, and be aware of any deadlines for the secondaries. Make sure any additional letters of recommendation are received before the deadline. Some people take much longer than they claim to write the letters. It is not their responsibility to make sure the letters are in on time, it is yours! Most schools will inform you when your application is complete. If you do not hear from the school within two weeks after you feel all materials should have arrived, contact the school to make sure your file is complete.
After all information is received by the school, your application will be reviewed to determine whether you will be granted an interview. Procedures for reviewing applications and notifying applicants of interviews and acceptances vary from school to school. The secondary application may explain what to expect; if not, contact the schools and familiarize yourself with each school's procedures.