International students may not volunteer to work a job that is normally a paid position, nor are they allowed by immigration regulations to trade a service (i.e. babysitting, yard work, house-sitting, etc.) in exchange for any type of compensation (i.e. food, housing, small gifts, etc.). Doing so is illegal and grounds for termination of student status.
For the purposes of F-1 and J-1 student status requirements, students should consider any activity to be "employment" if any type of compensation is exchanged for performing a task or providing a service.