Most US M.D. medical schools participate in the AMCAS, or the American Medical College Application Service. Using this service, you can apply to almost all M.D. medical schools with one application. In other words, you fill out the AMCAS, and check off which schools you want AMCAS to send your application. Every US M.D. school outside of Texas participates in this service. Only some medical schools in Texas use this service. Learn more about the AMCAS.
US D.O. medical schools participate in the AACOMAS, which is the D.O. equivalent of the AMCAS. Again, you fill out toe AACOMAS and choose which schools to send the application out to. Learn more about the AACOMAS.
Also see: CASPer Exam for Medical School
Medical schools outside of the US might have different application processes.
Note that medical schools have primary applications and secondary applications. The AMCAS and AACOMAS are used only for the primary applications. After respective schools receive a student’s primary application through these services, schools will send out a secondary application specific for that particular school.