OPT (optional practical training) allows graduates (not just STEM majors) to work for up to one year in a job related to their major. STEM graduates can get an additional 24 months - allowing them to work up to 36 months in total after graduation. After that, they will need to find a sponsoring employer if they want to continue working in the US.
It actually does not matter what a college thinks is STEM vs not. To be eligible for the STEM extension, the major must be designated as STEM by DHS. The list is here.