Well, I'll start the answers, although it's been a while. A commercial jet is operating to Performance Category A, which sets out all sorts of figures to achieve the safe flight. On approach the aircraft is assumed to cross the runway threshold at 50' at Vref (approach reference speed), with the speed reducing at a rate of 1kt/sec, and aiming for a touchdown within the touchdown zone markers (the big white markers some way beyond the threshold). This allows adequate margins for both stopping and going around. Military jets don't operate to the same regulations; I'm sure someone will come on soon and explain their side of things. I'm equally sure someone will correct my explanation, but it's a starting point.