What Is Zina?
Question

I need further information about the verse interpreted here about adultery. Adultery, according to the Hudood I read, requires penetration. Is that true of Islamic law?
Are married persons, or single for that case, indulging in immoral acts that do not include penetration (but maybe oral sex), exempted from this punishment?
Secondly, is there forgiveness for adultery? Or for what I inquired in my previous question.
I'd be greatly obliged to receive an answer, because this issue has confused me for a long time.
Praise be to Allah.What is Zina? 
Zina, in Arabic, means immorality, and is used with two meanings in Shari`ah: a general meaning and a specific meaning.
The general meaning includes that which carries the punishment (hadd) and that which does not carry it. Islam does not give the name of Zina only to that which carries the punishment, which is just one of many types of Zina. 
Ibn ‘Abbas (may Allah be pleased with them both) said: I have never heard any better definition of small faults [al-Najm 53:32] than that which Abu Hurayrah (may Allah be pleased with him) narrated from the Prophet (peace and blessings of Allaah be upon him): Allah has decreed for every son of Adam his share of Zina, and there is no way to escape from it. The Zina of the eye is a glance, the Zina of the tongue is speaking, and the Zina of the mind is wishing and hoping; then the private part either acts upon this or it does not. (Reported by al-Bukhari, 11/26; Muslim, 4/2046).
Al-Bukhari included this hadeeth in a chapter entitled Bab Zina al-jawarih doon al-farj (Chapter on the Zina of faculties other than the private part).
Al-Hafiz Ibn Hajar (may Allah have mercy on him) said: Zina is not always associated with the private part itself, but may involve other faculties such as the eyes and so on. Ibn Battal (may Allah have mercy on him) said: Looking and speaking are called Zina because they lead to real Zina. This is why he said that the private part either acts upon this or it does not
Zina: A Grave sin in Islam
Zina is one of the forbidden deeds, one of the most serious major sins (kabair) after shirk and murder. Allah says (interpretation of the meanings):
“And those who invoke not any other god along with Allah, nor kill such life as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace; except those who repent and believe and do righteous deeds, for those Allah will change their sins into good deeds, and Allah is Oft-Forgiving, Most Merciful.” [al-Furqan 25:68-70]
“And come not near to unlawful sexual intercourse. Verily, it is a fahishah [anything that transgresses its limits a great sin], and an evil way (that leads one to Hell unless Allaah forgives him).” [al-Isra 17:32]
Imam al-Qurtubi (may Allah have mercy on him) said: The scholars said that the phrase ‘And come not near to unlawful sexual intercourse’ is more eloquent than merely saying Do not commit Zina, because the meaning is, Do not even come close to Zina. This means not doing any deed that may get close to Zina or lead to it, such as being alone with a member of the opposite sex, touching, looking, going to evil places, speaking in a haraam manner to a woman to whom one is not related, thinking about and planning immoral acts, and so on.
Abd-Allah ibn Masood said: I asked the Messenger of Allah (peace and blessings of Allah be upon him), Which sin is worst in the sight of Allah? He said, To make any rival to Allah, when He has created you. I asked, Then what? He said, To kill your child for fear that he will eat with you. I asked, Then what? He said, To commit Zina with the wife of your neighbour. (Reported by al-Bukhari, 8/492; Muslim, 1/90).
All religions are agreed that Zina is haram; no religion allows it. The punishment for Zina is the most severe of punishments, because it violates peoples’ honour and lineages.
Degrees of zina
The sin of Zina may be of varying degrees, and the gravity of the offence depends on the nature and circumstances of the deed, although all Zina is haram, a major sin and an act of immorality. Zina with a mahram [i.e., incest] (Allah forbid) or with a married woman is far worse than Zina with a woman to whom one is not related or with an unmarried woman, because it involves violating the rights of the husband, contaminating his bed, attributing a child to him that is not his, and other kinds of offence and harm. 
If her husband is a neighbour, the crime of being a bad neighbour is added to the offence. If her husband is one's brother or relative, the crime of breaking family ties is added. The Prophet (peace and blessings of Allah be upon him) said: He will not enter Paradise whose neighbour is not safe from his evil actions. (Reported by Muslim, 1/68). 
There is no evil action worse than Zina, and if the husband is absent for the sake of Allah, such as in worship, seeking knowledge or jihaad, then the sin is compounded. Buraydah (may Allah be pleased with him) reported that the Prophet (peace and blessings of Allaah be upon him) said: The wives of the mujahideen are as sacred and forbidden to those who stay behind as their own mothers. There is no man of those who stay behind who is entrusted by one of the mujahideen with the care of his family and then betrays him (by committing Zina), but he will be detained on the Day of Resurrection, and (the one whom he betrayed) will take as much of his good deeds as he wishes. So what do you think? (Reported by Muslim, 3515)
Punishment for zina
If the adulterer is married, the sin is greater, and the punishment is stoning, and if he is an old man, the sin is greater and the punishment is as the Prophet (peace and blessings of Allah be upon him) mentioned in the hadeeth about the three whom Allah will neither look at nor praise on the Day of Resurrection, and who will have a painful punishment. (Saheeh Muslim, 156)
If the deed is also committed during a sacred month or in a sacred place or at a time which is special in the sight of Allah, the sin is compounded.
What constitutes zina?
The jurists (may Allah have mercy on them) stated that the basic act of Zina which carries the punishment is illegal sexual intercourse, whereby the two circumcised parts [i.e. genitals] come together and there is penetration of the tip of the penis, because this is actual penetration (which carries the prescribed hadd or punishment).
What leads to zina?
A person must avoid evil deeds and everything that can lead to them. Allah has commanded us to avoid the traps of the Shaytaan, because if a man takes one step in that direction, his shaytan and his own ego (nafs) will keep urging him to do evil until he commits immoral deeds.
One should think about how serious it is to touch a woman to whom one is not related, as the Prophet (peace and blessings of Allaah be upon him) explained: If one of you were to be stabbed in the head with a piece of iron it would be better for him than if he were to touch a woman whom it is not permissible for him to touch. (Reported by al-Tabaraani; see also Saheeh al-Jaami, 5045). 
This refers to the punishment for touching, so how about worse deeds, such as embracing and kissing, and even worse kinds of illicit activity? The Muslim judge is empowered to stipulate an appropriate punishment for every proven action other than actual intercourse; in the case of intercourse, the punishment set out by Islam is one hundred lashes and a year’s exile for one who is unmarried, and stoning for one who is married. This is the punishment in this world, and the punishment in the Hereafter is far more severe.
Will adulterers be forgiven?
One of the important principles which the salaf (may Allah have mercy on them) pointed out is that the sincere believer who truly hopes for Allaah and the Hereafter should not look at whether the sin is major or minor, or whether is carries a punishment (hadd) or not. Ibn Abbas (may Allah be pleased with them both) said: There is no major sin if one asks for forgiveness, and there is no minor sin if one persists in repeating it. Someone else said: If you want to disobey Allah, do not think about the smallness of the sin, think about the greatness of the One Whom you are disobeying.
How can you seek repentance from zina?
With regard to the matter of repentance from Zina , please refer to the book "I Want to Repent, But..." May Allah bless our Prophet Muhammad.
For more about zina-realted issues, see these answers: 6926 , 649 , 5445 , and 2487
And Allah knows best.