I am talking about in the western tradition.
Monks can be priests.
The simplistic way of putting the difference are:
Monk is a devotional vocation.
Priest is an “administrative” vocation.(forgive me, that’s best I can do:))

Anything more than that is pretty complicated. For more reading about monks, you have to read the writing of St Benedict (in the west) or St Basil (in the east).

As for the monk in Buddhism, I am not an expert about it.

I guess you have some basic questions about Catholicism. I found a funny page may give you some ideas.
http://www.envoymagazine.com/backissues/5.1/airplaneapologetics.htm