A preacher is one called by God to go out and preach God’s Word to people.
A pastor is a preacher who has been called to be a preacher to a church. A pastor has one church that he preaches at and calls his “home” church.
A preacher might receive the call to preach, but doesn’t stay at one church all the time. He might be a member of a church, but he doesn’t necessarily pastor there. Think of the idea of circuit preachers, moving from town to town.