Most concerts in the U.S. are now seated concerts, which means that even on the floor they put folding chairs out and people are asigned seat numbers by their ticket. Nobody actually sits in them for the most part but security pretty much keeps you in the seats you have been asigned, now if there happens to be a few empty seats closer than you are, you can move to them, but if the original buyers of those seats show up you'll have to move back to your original seat.
Festival seating is when the floor is standing room only, which means there are no seats and your position in the crowd is only hampered by your ability to get to the front. These shows fell out of favor in the late 80's, early 90's due to a few crowd surges that killed a few people. So festival seating is usually only used in small venues these days.
One question from overseas... main floor seatting means people are supposed to be sitten just in front of the stage?