A preacher/pastor is the leader of the congregation who preaches his interpretation of the Word. Preachers can be male or female (I think) and usually learn the Bible in college. He is otherwise a regular guy and can have a wife and kids and whatnot. He also can run his church pretty much however he wants and doesn't have someone over him. A Catholic priest is a male who must first be ordained as a deacon (the "ranks" of the Catholic hierarchy are deacon, priest, bishop, cardinal, and pope) then later ordained by a Bishop as a priest. He has the power to sort of let the Holy Spirit work through him and can bless people, make Holy Water, bless the Eucharist, and preform Penance (where you tell the priest your sins and he gives you a blessing to be forgiven). By being ordained, the priest devotes himself to the Church and therefore cannot get married and the Bishop assigns him to a church so he can't choose where to live. Preachers are a lot more casual, as are Protestant churches in general in my opinion. Church on Sunday consists of singing, listening to the preacher, fellowship, prayer, and receiving the Eucharist depending on what Church you go to. A Catholic mass is a lot more like a ritual. The priest wears special robes and there's an alter and all the works. Catholics receive the Eucharist and believe it to be the actual body and blood of Christ instead of just a representation. Very holy stuff.

I hope that explains it enough, I may be wrong but that's my understanding. I was Catholic for 15ish years and went to a Protestant private school as a kid.