So, I'm a relative new guy to this guitar thing only having about a years experience.  However I've been heading out to a couple Open Mic nights.  My gear isn't amazing at all.  
Most importantly I only have a 40w small amp.  This is just enough for most of the small rooms I play.
The problem is that I'll be playing in a rather large auditorium tomorrow.  I have no idea if my amp will be enough by itself.
My question pertains to ways to deal with what I have and a professional PA system to make sure its enough.  Now, in a situation like this, should I try to take a plug from the headphone jack on my amp and run it into the PA system?
I've seen guys place a microphone in front of the amp itself, but I'm not too certain about why they do this.  I think this is for some sort of feedback effect, but I honestly don't know.  What's the purpose of this?