Final answer:
A germ-line mutation occurs in a sperm or egg cell and can be transmitted to offspring, while a somatic mutation occurs in other cells of the body and cannot be passed on to offspring.
Step-by-step explanation:
A mutation that occurs directly in a sperm or egg cell, or in one of their precursors, is referred to as a germ-line mutation. These mutations are significant because they can be transmitted to offspring, and every cell in the offspring will have the mutation. On the other hand, somatic mutations occur in other cells of the body and cannot be passed on to offspring.