Easy !
There are two student visas:
F-1: for academic studies
M-1: for non-academic or vocational studies (like CFR.61/141 training)
An F-1 student may not accept off-campus employment at any time during the first year of study; however, the INS may grant permission to accept off-campus employment after one year. F-1 students may accept on-campus employment from the school without INS permission.
Except for temporary employment for practical training, an M-1 student may not accept employment.
A student visa is required if you spend more than 18 hours/week on your studies/training.