Tahajjud = waking up before fajr to pray in 2 unit increments preferably last third of night. Excellent to strengthen emaan
Qiyam al Layl = Any prayer done after Isha before Fajr, don't have to wake up just pray it any time after isha, night time. Excellent if you afraid you will not wake up.
Tahajjud...wake up before fajr azan....and pray minimum 2 rakats and maximum upto 12 rakats......and 12 rakats must be in 2,2 rakat set...its mean u pray 2 rakats then 2 rakats then 2 rakats in this manner u perform 12 rakats..