Since you didn't really provide any specific questions, I can't really give you any specific information about the program. I am also unsure if you are looking to do the program at Walt Disney World or Disneyland. As the two are very different, the answers that I can give about WDW may not apply to Disneyland. Your best source for information is the official Disney College Program web site at
http://www.disneycollegeprogram.com.
Assuming that you are asking about the WDW program, I can answer the questions about extending. When you go on a program, you are allowed to extend one time and stay on for up to a year in total. For example, if you go on the Spring program (January to May/June) and decide to extend, you will be kept through the end of the Spring Advantage season, which ends in August. You won't be able to extend again, since you have used your one extension. If you go on the Spring Advantage program (January to August) and extend, you will stay through the end of the Fall program, which ends in January. As you will have both used your one extension and been on the program for a full year, you will not be able to extend again. If you want to go on the program again, you will need to wait a season before participating. At this point, your extension option will be available again. Keep in mind, though, that there is a program assessment fee (not sure how much) that will be charged if you extend and that you will likely have to move from one apartment to another. If you are stuck deciding between a regular semester program and the Advantage program, this is an important fact to consider. Also, if you like the program so much that you want to stay a whole year, you won't be able to if you didn't sign up for an Advantage program to begin with.
The professional internships
do count towards the one year limit, so be sure to factor that in if you are thinking of going from a regular program right to a professional internship. (So if you do a Spring Advantage program and extend for the Fall, you will
not be able to apply for a professional internship right away and will need to wait for the next Fall session.) Also, it is important to have a Plan B if you don't get accepted to any of the professional internships, as happened to me. To apply for a professional internship, go to
http://www.disneycareers.com.
If you need more information, just ask.