The VMCAS is owned and operated by the American Association of Veterinary Medical Colleges (AAVMC). The AAVMC coordinates the affairs of all veterinary medical colleges in the United States and Canada, including some departments of veterinary science, departments of comparative medicine, and seven international colleges of veterinary medicine. The VMCAS is the central application portal for all these entities. The VMCAS application that you will fill out is sent to all the schools to which you would apply.
The application portal usually opens in June of the year that you would apply (your rising senior year), and the application is due October. Note that this application is quite extensive and takes a lot of time to fill out, so start early! The VMCAS application will includes a comprehensive list of all your college courses and all your activities, going all the way back to high school.
It will also include a personal statement that will be limited in length. So you have to make this statement complete yet succinct. Therefore, take your time and invest a good deal of effort writing the statement. Have many people proofread it for you (including faculty, veterinarians with who you worked, and your mentors).
It also helps to get your letters of recommendation early, so don't dally!
So stay on top of everything and submit the application EARLY. Don't wait until the last minute to submit it.