What is the difference between a pastor, chaplain, minister?

I can only tell you the way it works in the churches with which I'm familiar. A person who is ordained but not affiliated with a particular church is a minister (or, more informally, a preacher). A minister (or preacher) who works at a particular church is often referred to as a pastor, or, if he or she isn't the senior pastor, an associate pastor, assistant pastor, or that sort of thing. Generally, a chaplain is a minister (or preacher) who works in the army, in a hospital or some other similar setting. I hope this helps.