Here's a breakdown:

* Embryo: The developing organism from fertilization to the end of the 8th week of gestation. During this stage, the major organs and body systems are formed.

* Fetus: The developing organism from the 9th week of gestation until birth. During this stage, the fetus continues to grow and develop, with organs becoming functional and the body preparing for life outside the womb.

So, while the term "embryo" refers to the early stages of development, "fetus" describes the stage when the major organ systems are established and the organism continues to grow and mature.