One-to-many: An entity in A is related with any number of entities in B. An entity in B is related with at the most one entity in A.
Example: Relationship between faculty and department.
One department can take on any number of faculty members but a faculty member is assigned to only one department.
Example: Relationship between course and instructor. An instructor can teach several courses but a course can be taught only by one instructor. Please note this is an assumption