When it comes to the men, I am always confused, but for the women here’s the deal-
All women religious are
sisters, but not all sisters are
nuns
.
The difference- a “sister” would live/minister/work in the community. She could be a teacher, nurse, social worker, etc. She would still be part of a religious community, and may or may not live with other sisters, but would be “obligated” to pray according to the charism of the order.
A “nun” (who is called ‘Sister’, which is why this is so confusing! :whacky

, is someone who “leaves” the world and lives in a cloister. Nuns live lives of prayer & contemplation, not so much in ways of “service”, except to each other in the convent.
So, to sum up, all nuns are sisters, but not all sisters are nuns!!