These are academic courses that last minimum one full academic year and leads to an academic qualification at the end. These courses cover all ages from 10 plus.
There are some academic pre requirements for these courses. You need to have certain level of language qualifications. You also need to have certain academic qualifications. You might need academic references.
These courses are minimum 2 weeks to 6 months. You usually do not need language qualifications for these courses. These courses can be vocational and might end to a qualification. You learn skills with these courses.
You would not need student visa for these courses. You might be able to extend the course at the end depending on the immigration rules. In UK you cannot extend the short course length beyond your visa period but in USA you can.