I-20 form (see the sample image below) is a document that serves as a precursor to obtain a F1-student visa. International students willing to study in US, need to have an I-20 form before he/she can get a student visa.
I-20 form (see the sample image below) links the student and the university which has granted admission to him/her.It identifies the student,the school he is hopping to attend, the degree sought,intended major, English proficiency of the student, student's country of origin and the cost of attending the school. It will clearly show how the financial obligations of the student (expected expenses during the course of study at-most for 1 year period) will be met. The form also includes the student tracking number (SEVIS number - "Student & Exchange Visitor Information System" number) for the student and program
Pre-requisite for an I-20:
1) Acceptance to a program (undergrad or grad course) from an university/school
2) If the school requires "English Proficiency", then the student must have demonstrated his "English Proficiency" by some means ( ex: TOEFL,IELTS scores etc..,)
3) Cost of attending the university/school (estimated upto maximum 12 months)
4) Information on how the student will be supported financial for the above mentioned period (by grants, scholarships, fellowships,self support,support from family etc..)
After obtaining I-20:
The university which accepts the student will fill-up the I-20 with the above mentioned information and will send it to the student by post. Once the student receives the I-20, he/she needs to sign it. To obtain a F1-student visa, the student needs to go to a nearby US consulate/embassy (with prior appointment and DS160 form) with a valid passport and the completed I-20 form.
List of US consulates and their addresses around the world can be obtained from here.
The US consulate will issue a F1-student visa (which will be stamped on your passport). Then you can go ahead and purchase your plane tickets.
Landing in the US: (I-94 card)
After you have landed your feet on the US soil, the first form you will fill-up is the I-94 form (This is also called arrival-departure record card/form - see sample image below). It will record your date of arrival and your duration of stay in the US. You may remain in the US till the departure date marked on the I-94.
Note: I-20 is NOT applicable for international students who are wishing to enroll in a primary/secondary school in United States.
Sample I-20 Form:
Sample I-94 form: