What is the difference between Witr and Qiyam Al-Layl, do they differ in regular days from Ramadan, and what is Salat Al-Tahajjud?

There is no difference between Tahajjud, Qiyam al-Layl, and Iḥyā’ (reviving the night in prayer); all these terms refer to voluntary prayers performed after the 'Ishā' (night) prayer. This is a confirmed Sunnah at all times, becoming even more emphasized in Ramadan. Witr prayer is the closing of Qiyam al-Layl with a single rak'ah, based on the Prophet's (peace be upon him) saying: "Make the last of your prayer at night Witr." Whoever prays only Witr has missed Qiyam al-Layl.