Nuns are cloistered, meaning they don’t leave their convent. Sisters aren’t cloistered, so they study or teach or do some other work outside their community.
It’s the same with monks and brothers (also known as friars): monks being cloistered and brothers/friars not.
**All of the above terms refer to people who are in a religious order.
A priest is a man who has been ordained by a bishop, and he has the faculties to celebrate Mass, hear confessions, give homilies, etc. There are both diocesan (or “secular”) priests and religious priests. The religious priests are members of religious orders, and the diocesan priests are not.
So, to be clear (with saint examples), a man could be simply a diocesan priest (St. John Vianney) OR a monk and a priest (St. Jerome*) OR a friar and a priest (St. Thomas Aquinas) OR simply a monk (St. Peter Damian) OR simply a friar (St. Francis of Assisi).
*St. Jerome wasn’t in any established religious order, but he did live as a hermit, and he was the best example I could think of. So there.