A little more detail:
The primary/AMCAS (same thing) is the application that includes your personal statement, list and description of extracurriculars/work/honors, and grades from school. I believe you can submit the AMCAS anywhere from June-October or November, depending on the school, but it's best to submit it as soon as possible (June or July). The same AMCAS application is sent to all the schools that you designate on the application website. After you submit the AMCAS, your grades must be verified, which could take days or weeks.
For the secondary round, some schools will request more information from you and give you a secondary application. Some schools give the secondary to all applicants; other schools "screen" you first by looking at your AMCAS and deciding whether or not they want to continue considering you as an applicant. Secondaries typically involve more essays, a fee, and a request for letters of recommendation. Sometimes there are no essays. Secondaries are unique to each school. You could get secondaries as soon as your AMCAS is submitted, if the school does not screen, or you may not get secondaries until after your AMCAS is verified.
Hope that helped!