A student visa (typically F-1 or J-1) is required to study in the US. A lot of attention should be paid to the process of applying for a US visa. The applicants should be able to demonstrate their financial assets to cover the education and the willingness to return to their home country. Make sure you have a passport before you apply for US Universities. Even if your academic record is great, or you have an admission offer from a top University, the visa application process should be taken very seriously. Hope for the best and be prepared for the worst. Take all the required documents with you, as suggested on the website of the US Embassy or Consulate of your country. Our forums have a dedicated thread on visa related discussions and information sharing.
Understanding the concepts of VISA and STATUS
Suppose a tourist is going to USA on a B1 visa. She applied for a 60 days visa, and for whatever reason the visa was granted for 1 year on February 15, 2008. Now she is eligible to enter the USA on any date between February 15 2008 and February 14, 2009. Now at the port of entry on March 03, 2008 the INS/DHS officer after talking to the tourists stamps the passport and I-94 with remarks that the stay is permitted for 90 days starting the date of entry expiring on June 03, 2008. Now this tourist must leave the US within those 90 days, even if the visa may be valid for next 2 days or the next 200 days.
Based on the above example try to understand that the role of visa is over when you enter the US. What keeps you legally in the US is your 'status'. The above tourist was granted a B1 status for 90 days. Most other visas including the H1B visa have a fixed number of days or year. But the student status is unique in the sense that instead of an expiry date the INS officer on the port of entry writes "D/S" on the passport and on the I-94. These magical words mean 'Duration of Status' and make you eligible to stay in the US till you maintain your student status*. If you find that the visa officer has written any expiry date, (even if 2 or 3 years from the date of entry) politely ask her why D/S was not given. If the status of D/S was not given, bring this to the attention of your international student advisor at the first opportunity. A trip to the nearest DHS office may be required.
* maintaining your status is a complicated topic in itself. For simplicity, it means you are in status as long as you are enrolled for the intended degree program with minimum course load, making progress towards the degree and not indulging in unlawful activities like working off campus.