The Qur'aan is the word of God which was revealed to the Prophet Muhammad, may the peace and blessings of God be upon him, and preserved both verbally and in writing by his Companions. Apart from the Qur'aan, whatever the Prophet uttered or did was also preserved by the Companions. Thus the sunnah includes the sayings of the Prophet, peace be upon him, known commonly as ahadith (i.e. sayings), his practices, and actions which gained his approval. Both the Qur'aan and the sunnah fall under a common title - wahy (i.e. revelation or inspiration) 
Thus as Muslims, we are required to believe that the Qur'aan and Sunnah go hand in hand together as our sources of legislated law. Both are revelation from Allah, the most High. The Qur'aan is composed of the actual words of Allah, whereas the Sunnah is expressed through the words, actions and approvals of the Prophet Muhammad. Another difference is that the Qur'aan is recited formally in the prayers whereas the Sunnah is not.
As a way of life, Islaam is perfect and complete. However, such are the times that we live in, that some of the liberal elite from the Muslims choose to deny aspects of Islaam to suit their own desires. It is even more sad, that many choose to deny the Sunnah in particular. However this is clearly wrong, as the saying of the Prophet indicates: “I have been given the Qur'aan and something similar to it besides it. Yet a time will come when a man leaning on his couch will say ‘follow the Qur'aan only; what you find in it permissible, take as permissible, and what you find as forbidden, take as forbidden’. But verily what the Messenger of Allah has forbidden is like what Allah has forbidden” 
Linguistically, Sunnah is an Arabic word which means a path or a way. The Sunnah (السنّة) in its linguistic meaning came in different definitions that all pour into one meanings. The author of Lisaan al-Arab defined it to mean: "The way, the method, the regular habits, the nature". These various lingual definitions seem to require a form of continuation to the act so that it be considered a Sunnah.
Also, it is important to note that the term can apply on anyone and is not restricted to the Prophet (صلىالله عليه وسلم). So, we can say the Sunnah of life, the Sunnah of X and Y, the Sunnah of Son of Adam, the Sunnah of Muslims and etc. However, if this term was used to refer to the Prophet (صلى الله عليه وسلم) then an additional meaning is offered here and the context differ.
Islaamically, it is a primary source of law taken from the sayings, actions and approvals of the Prophet Muhammad. As Muslims we believe that the law Prophet Muhammad came with, is a divine revelation from our Creator, Allah. However many people believe that the Qur'aan is the only form of divine revelation, as it is the literal word of Allah. This view is incorrect as it contradicts the Qur'aan itself. About the Prophet Muhammad, Allah says: “Your companion (Muhammad) is neither astray nor being misled. Nor does he speak of his own desire. It is (only) the revelation with which he is inspired” (Surah An-Najm 53:2-4).
The Sunnah to be either obligatory or recommended. al-Subki defined the Sunnah to be: "The Sunnah in Istlaah (religious context) means: what have been known as obligatory or preferable from the saying of the Prophet" [al-ibhaaj fi sharh al-minhaaj: 1/36] 
Before the advent of the Prophet Muhammad, different nations on this planet had received a Messenger. The purpose of these Messengers was to instruct their people in how to worship Allah correctly. The Prophet Muhammad was no exception to this. If we had been given the Qur'aan on its own, then most of us who are Muslims would have gone astray through our own interpretations of this book. Rather, Allah through his mercy, sent a noble Messenger who came to explain this Qur'aan so that the people of the earth may understand their Creator better: “We have revealed the reminder (Qur'aan) to you (O’ Muhammad) in order that you explain to the people what has been revealed to them, that perhaps they may reflect” Qur’an.Surah An-Nahl 16:44.
By explaining the Qur'aan through his words and actions, the Prophet through his Sunnah, gives us the best example of how this book is to be understood and practiced. This is why Allah says: “Indeed in the Messenger of Allah you have the most beautiful pattern of conduct” (Surah Al-Ahzab 33:21). Our need for the Sunnah is so great, that without it our religion would be incomplete. As Muslims we do not have this worry, because the Sunnah is our divine explanation. The Qur'aan commands us to worship Allah through prayer, fasting, charity etc., but it does not inform us of how to perform these duties practically. It is through his infinite Wisdom, that Allah has sent us a practical example in the form of the Prophet and his teachings. The Prophet's adherence and practice of the Qur'aan was so exemplary, that upon being asked about his character, his wife Aisha said: “…his character was that of the Qur’aan” Muslim - Eng. Trans. Vol 1., Pp358-360, No. 1623.
Obeying Allah is without doubt, obligatory. So when Allah says: “Whosoever obeys the Messenger, has indeed obeyed Allah” (Qur’an.Surah An-Nisa 4:80), it should be clear that one has obeyed Allah by obeying the Messenger. Furthermore Muhammad said: “…whosoever disobeys me, disobeys Allah”Bukhari - Eng. Trans. Vol 9., p.198, No. 251.. Following the Sunnah is clearly an obligation upon every Muslim.
The obligation is stressed even more when Allah says: “But no, by your Lord, they can have no faith, until they make you (O’ Muhammad) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept them with full submission” Qur’an.Sura An-Nisa 4:65
“It is not fitting for a believer, man or woman, when a matter has been decreed by Allah and His Messenger to have any choice in the matter. If anyone disobeys Allah and His Messenger he is clearly astray” (Surah Al-Ahzab 33:36). As Muslims we know that when Allah or His Messenger decree something for us, it will always benefit us, even if we do not realize it. Thus, submitting to the Messenger is only there for our benefit and not to oppress us, as some mistakenly believe.
It must also be remembered that besides being the Messenger of Allah, Muhammad was an Arab man living in seventh century Arabia. This meant that he had his own personal tastes and preferences just like any other mortal. These preferences are clearly distinguished from the Islamic law which is binding upon everyone. Thus, his personal Sunnah is clearly distinct from the legal Sunnah which he brought. An example of this distinction is when the Prophet came to Madeenah and found the inhabitants artificially pollinating the date palm trees. When he asked them why they did so, they replied that it was their habit. He suggested to them that maybe if they did not do it; it may be better. So they gave it up and the following year their crop was greatly diminished. When they told him of this he replied: “I am only a human being. When I issue any command to you regarding your religion, then accept it, but when I issue any command to you based on my own opinion, I am merely a human being” Muslim. Eng. Trans. Vol 4., p.1259, No. 5831.
Islaam is a religion which singles out Allah alone for worship. It is for this reason that the possibility of Muhammad being anything more than a man, is totally absurd. As him being the Messenger of Allah we believe that his Sunnah is infallible, but at the same time we maintain a perfect balance by remembering the words of Allah: “Say (O’ Muhammad) : ‘I am only a man like you (except that) it has been revealed to me that your God is one God” (Surah Al-Kahf 18:110). True Muslims will follow Muhammad, not worship him.  http://www.islaamnet.com/whatissunnah.html
One of the scholars of the past, Imam Malik, said: “The Sunnah is like the ark of Noah. Whoever embarks upon it achieves salvation and whoever rejects it, is drowned”Quoted by Shaikh ul-Islaam Ibn Taimiyyah in Majmoo' ul-fataawaa (4/57)
This salvation is the admittance into Paradise and avoiding the fire of Hell. Regret is a terrible state. But regret on the Day of Judgment will be even worse for the one who did not follow the Sunnah: “And remember the day when the wrongdoer will bite at his hand and say: ‘Oh! Would that I had taken the path of the Messenger’” (Surah Al-Furqan 25:27). This regret will continue during the punishment: “On the day the faces will be tossed about in the fire, they will say: ‘Woe to us! Would that we had obeyed Allah and his Messenger” (Surah Al Ahzab 33:66). On the other hand, the one who adhered to the Sunnah will attain the ultimate benefit: “Whoever obeys Allah and his Messenger will be admitted to gardens beneath which rivers flow to live there (forever), and that will be the great achievement” (Surah An-Nisa 4:13). This is further confirmed by the Prophet himself when he said: “He who obeys me enters paradise, and he who disobeys me refuses to enter paradise” Sahih Al Bukhari - Eng. Trans. Vol 9:384
The sweetness of the Sunnah will also be tasted during this life. The vastness of the Prophet's way is such, that is produces a physical, spiritual and psychological benefit to its adherent. This fact is also recognized by non Muslim scientists who have discovered that the Sunnah is extremely accurate in its conformity with modern scientific data. Scientific and Medical facts which were recorded more than 1000 years ago, from the tongue of the Prophet, have only been recently discovered. Such findings prove that the Sunnah could only have been divinely revealed. Thus, the one who lives his life by this law will bring goodness upon himself and upon the rest of this decaying world. As for one who chooses to discard these divine rules, then that is a person who has lost a treasure which far outweighs any treasure that the earth can bring forward. A treasure which will bring eternal happiness.
And the one who reforms his life around the Sunnah of Muhammad, can be assured of a complete guidance:
"I have left two matters with you. As long as you hold to them, you will not go the wrong way . They are the Book of Allah and the Sunna of His Prophet.
Muwatta Malik, the book of Decree Book 46, Hadith 3 (Arabic: Book 46, Hadith 1626) " رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " تَرَكْتُ فِيكُمْ أَمْرَيْنِ لَنْ تَضِلُّوا مَا تَمَسَّكْتُمْ بِهِمَا كِتَابَ اللَّهِ وَسُنَّةَ نَبِيِّهِ "
   http://www.islaamnet.com/whatissunnah.html