Answer:
The two words can be used interchangeably in everyday conversation (I have heard both 'laundry soap' and 'laundry detergent' used). Chemically though,"Detergent" refers to any surfactant (a chemical that lowers the surface tension of a liquid) that has cleaning properties and soap refers specifically to the salt of a fatty acid. Therefore soap is a form of detergent, but not all detergents are soaps.