What is the Form I-20, who issues it, and when do you need it?

The Form I-20 is the Certificate of Eligibility for Nonimmigrant Student Status, required for every F and M student. After you are admitted, a designated school official (DSO) at the school issues it in SEVIS; you do not apply for it yourself.

You will use it three times in the process. The SEVIS ID on it is needed to pay the I-901 fee, the original goes to the visa interview, and the signed form is shown to the CBP officer at the port of entry, so it travels in your hand luggage, not a checked bag. It is not finished with after that either: a driver's licence office or the Social Security Administration will ask for it as proof that you are a student in status, so keep it for the whole of your program.

Both you and the DSO sign it, and if you are under 18 a parent signs too. If you bring dependents, each F-2 gets a separate I-20 of their own, issued by the school.