A F-1 student can use two types of practical training: curricular practical training (CPT) and optional practical training (OPT), each for up a total of 12 months. Both CPT and OPT can be taken multiple times, as long as when you have not reached the 12-month limit. For example, you can take CPT once for 3 months, once for 6 months, and another time for up to 3 month.

CPT is usually used for internship before you graduate from school. It's called curricular practical training because it's regarded as a special course being taken. You need to prepare a "course" description, get approval from your department (essentially your advisor, if you have one). You also need to actually register for a course and pay the tuition. CPT application can be approved by the international student office on your campus. It's tied with a particular employer for a specified time period.

OPT is usually used after a foreign student graduates from school. You have up to 60 days after graduation to apply for it. You can specify the starting time and the ending time. Usually you cannot apply for it again after you have graduated for a year or longer. OPT is not tied to any particular employer.

Question: Once I get a job after graduation, should I use up my OPT first, or should I apply for H-1 visa immediately? Using OPT first has two advantages: you can change jobs if you want, and you don't pay social security tax. But given the fact that H-1 visa quota is often used up the first 6 months of the year, you probably don't have much choice but to apply for H-1 visa as soon as possible, if you want to continue working in the US after your OPT period.

Therefore, before you graduate, apply for OPT. Once you have OPT, you don't need a H-1 visa to start a job. Once you get a job offer, apply for H-1 visa immediately (you need sponsorship from your employer for H-1 application). While you wait for you H-1 visa, your OPT status allows you to continue working legally. Once your H-1 visa arrives, your OPT status expires automatically.