Adultery is a great sin according to the Quran. God, the Exalted, says: "Nor come nigh to adultery: for it is a shameful (deed) and an evil, opening the road (to other evils)." In this short verse, three important points have been mentioned:
A) The verse says "do not get near adultery" which not only incorporates an emphasis but it also connotes that the evil of adultery normally has certain preliminaries which drives a man to adultery gradually. An unlawful gaze, nudity, immodesty, reading sex stories, watching porn movies, bad company, privacy with a non-mahram (a man's being together with a woman in a private place) and finally not taking action for marriage and unreasonable restrictions imposed by both sides in this regard serve as preliminaries leading a person to adultery. The aforementioned short verse forbids all of the above in an implicit manner. However, the Islamic traditions forbid each one of them separately.
B) The sentence "إِنَّهُ کانَ فاحِشَةً" [verily, it is a shameful (deed) and an evil] consists of three emphasis (The term 'verily' plus the past tense of the verb 'kana' and the term evil) makes further tangible the greatness of this tangible sin.
C) The sentence "ساءَ سَبِیلًا" [it is a bad way] indicates that this evil act opens the road to other evils in society."
In any event, both zina (which is intercourse taking place between a man and woman who aren't married (permanently or via mut’ah) and the other acts that lead to it and are usually done before it are great sins, and the practical difference between the two is that in the former (zina), the Islamic governor or judiciary can carry out the hadd on the fornicating individual (which is a punishment specified in the Quran). On the other hand, if the man and woman haven't committed zina and what they have done are other unchaste acts between each other, they have still sinned and the judiciary can punish them; the punishment’s harshness depending on the greatness of what they have done (this type of punishment is called ta’zir).
 - Isra (Night Journey), 32, وَ لا تَقْرَبُوا الزِّنى إِنَّهُ کانَ فاحِشَةً وَ ساءَ سَبِیلًا As for the prohibition of adultery/fornication, see: Al-Furqan, 68 and 69; Al-A'raf, 33; Al-An'am, 151; Kulayni, Ya'qub, translated by Mustafavi, Sayyid Jawad, Usul-e Kafi, vol.3, pg. 391, Wafa Publications, 1382 (2003); Najafi, Muhammad Hassan, Hurr Amili, Wasail al-Shi'ah, vol.28, Kitab al-Hodud, Jawaher al-Kalam, vol.41, pg. 260 and 258, Dar Ihya al-Turath al-Arabi, Lebanon, 1981.
 - Tafsir Namuneh, vol.12, pg. 103.
 - Nur:2 “الزَّانِیَةُ وَ الزَّانی فَاجْلِدُوا کُلَّ واحِدٍ مِنْهُما مِائَةَ جَلْدَةٍ وَ لا تَأْخُذْکُمْ بِهِما رَأْفَةٌ فی دینِ اللَّهِ إِنْ کُنْتُمْ تُؤْمِنُونَ بِاللَّهِ وَ الْیَوْمِ الْآخِرِ وَ لْیَشْهَدْ عَذابَهُما طائِفَةٌ مِنَ الْمُؤْمِنینَ” (As for the fornicatress and the fornicator, strike each of them a hundred lashes, and let not pity for them overcome you in Allah's law, if you believe in Allah and the Last Day, and let their punishment be witnessed by a group of the faithful).