The M-1 visa is available to people coming to the U.S. for vocational study. (This visa is authorized the the U.S. Immigration and Nationality Act; see I.N.A § 101(a)(15)(M), 8 C.F.R. § 214(m).) There is no limit on the number of people who can receive student visas in any one year.
You might not need to go through the whole process of applying for a student visa, however. If you come to the U.S. as a tourist, it's okay to take a class or two for recreational purposes. Similarly, people who have a spouse or parent in the U.S. with an A, E, G, H, J, L, or NATO visa or status, or workers in H status can attend school so long as they don't do anything that interferes with the other terms of their visa or nonimmigrant status.
Here are some of the pluses, minuses, and important considerations for holders of an M-1, vocational student visa:
To qualify for an M-1 vocational student visa, you first must have been accepted at a school that has received advance approval from the U.S. government. You must be coming to the U.S. as a bona fide student pursuing a full course of study. That means you must attend at least 18 hours per week, if the courses consist mostly of classroom study; or if the courses mostly laboratory work, 22 hours per week. Your intended school program must lead to a vocational objective, such as a diploma or certificate.
You must also already be accepted by the school of your choice and have enough money to study full-time without working. The U.S. consulate deciding on your visa will ask you to prove that you have enough cash on hand to cover all your expenses for up to a year.
You must be able to speak, read, and write English well enough to understand the course work (and, as a practical matter, the consular officer who interviews you for the visa, who will be informally testing your English ability). Alternatively, the school can offer special tutoring or instruction in your native language to help overcome any linguistic barriers.
In addition to proving you have the necessary background and financial capacity to pursue your studies, you must prove that you intend to return to your home country when your program is over. This is the same requirement for almost all nonimmigrant (temporary) visas (except a few that allow "dual intent"). If you are going to look for a way to take up permanent residence in the U.S. when your visa runs out, you are legally ineligible for an M-1 visa.