Salat Tahajud is night Prayer when you wake up and start praying when every one is sleeping. its some called Salatu laili prayer of the night. Salatu Tahajudi is conducted not loud not quiet, its in between, since people are sleeping you must not read so loud , and you must not read so silent, you have to be in between. its may be prayed lonely or in congregation a minimum of two Rakaat and maximum of any number as you may wish.
Time of this Prayer
This special prayer not mandatory is prescribed to be conducted during night when people are sleeping. this means its a condition you must sleep and Wake Up to perform this Prayer.That means we if you sleep earlier and wake up at 10 ocklock you can start praying this prayer until before dawn. Some they pray this prayer together with Witr, all together they make it eleven Rakaats.
The best time to perform this prayer as toward the end of the night close to dawn before Salatul Fajr so that you do not sleep after waking up until fajr salah.