Well the first difference is that PMS is mostly a psychological and somewhat physical syndrome. The proof is that the symptoms of PMS are irritability, stress and dysphoria. There is of course a lot of other symptoms like anxiety, insomnia, headache, fatigue, mood swings, and increased emotional sensitivity. The cause of PMS are the changes in levels of estrogen and progesterone. Now, menstrual cramps are physical condition. The symptom is pain in lower abdomen. And the cause is the menstruation itself. So there you have the differences. PMS as its own name says (Premenstrual syndrome) occurs before menstruation, more accurately 5 - 10 days before menstruation which is luteal phase of menstrual cycle. Menstrual cramps can occur several days before menstruation or can occur when menstruation starts, and they slowly disappear as menstruation slowly ends
