A contract of service is an agreement whereby one person agrees to employ another as an employee and the other agrees to serve his employer an an employee. The employer would need to contribute statutory retirement pension / fund and provide relevant statutory benefits such as annual leave and sick leave.
A contract for service is an agreement whereby a person is engaged as an independent contractor, such as a self-employed person or vendor engaged for a fee to carry out an assignment or a project for the company. Under such an arrangement, there is no employer-employee relationship, and the person is not covered by a country Employment Act.
(source: Guide on Employment Laws for Employers, Singapore MOM April 2010)