Any form of sexual relationship, let it be speaking intimately, touching and caressing etc. before marriage is haram. As for fornication or zina which is intercourse with one who isn't your legal wife (by permanent marriage or mut’ah), it is a great sin as mentioned in the Quran, and there are several conditions for it, one of them being actual penetration. Therefore, as long as no penetration has taken place, the term zina will not apply, although as was said, any form of sexual relationship is forbidden and considered a great sin in Islam. In order to make up for this sin, one must regret what has been done and repent to Allah (swt). Also, although such a sin has taken place between the two, it still won't get in the way of the validity of your marriage if you ever want to get married.
Islam has introduced marriage as its answer to the needs of men and women, forbidding all other types of sexual relationship between the two genders, let it be intimate speaking, touching and caressing etc.; all of these can only happen after the marriage formula has been executed. Even a boy and girl who are fiancés and are planning to get married in the future, but haven't actually carried out the marriage formula cannot take lustful pleasures in each other, even if it is only limited to intimate speech.
As for zina or fornication, which means to have intercourse with a person who isn't one’s spouse (either by permanent marriage or mut’ah), it is a great sin in the Quran’s view about which Allah (swt) has said: “Do not approach fornication. It is indeed an indecency and an evil way”. There are several conditions for an act to be considered zina (which is the subject for numerous rulings such as hadd, or eternal impermissibility of getting married to that individual which applies in some cases) , one of those being for penetration to take place until the glans has been completely enveloped, and until such hasn’t taken place, it isn't deemed zina, although all forms of sensual relationship between non-mahram men and women are haram and a great sin. 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). Nevertheless, the punishment mentioned in the Quran won't apply anymore. If one ever wishes to make up for such a sin, all that is needed is for him/her to be truly penitent about it and to ask for repentance from Allah (swt). Also, if these two individuals ever plan on getting married, the fact that they had engaged in such acts before their marriage won't hinder the validity of their marriage.
For further information, see:
1- Question, 671 (website: 717), The ruling of sexual relationship before marriage contract.
2- Question 1222 (website: 1219), The chatting of boys and girls.
3- Question 695 (website: 743), Temporary marriage, the best solution.
4- Question 1407 (website: 1427), Having a relationship with a non-mahram before marriage.
5- Question 3012 (website: 3273) Zina with a married woman.
6- Question 2569 (website: 2723) The ruling on unchaste acts (Zina and others).
 Adopted from Question 1407 (website: 1427): Having a relationship with a non-mahram before marriage.
 Isra:32 “وَ لا تَقْرَبُوا الزِّنى إِنَّهُ کانَ فاحِشَةً وَ ساءَ سَبیلا”.
 Adopted from Question 2855 (internet user): Zina with a virgin girl and marriage to her.
 Adopted from Question 3012 (website: 3273): Zina with a married woman.
 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).
 Adopted from Question 2569 (website: 2723): The ruling on unchaste acts (zina and others).