Among all the different disciplines discussed here, the way the jurists use the word Sunnah is closest to its lexical definition. This usage, meaning "a praiseworthy way of action," has actually turned out to be a source of confusion concerning the status and importance of the sunnah. 
Jurists, for the most part, are concerned with the rulings of particular actions. In general, an act may be classified into one of five categories: obligatory, recommended, permissible, reprehensible or forbidden. In addition, an act may be sound and valid or it may be void and non-effective.
The scholars use a myriad of terms to describe the category of recommended acts. These terms included mandoob مندوب, mustahabb مستحبّ, and so on. In some cases, each term has a slightly different connotation.  However, without a doubt, one of the most common terms used for that category is the word "sunnah." Therefore, for example, the jurists will say that the two rak'atayn before the the obligatory Fajr (Dawn) prayer are "sunnah." This means that they are not obligatory. Yet they carry a certain status or reward for them such that they are definitely more than merely permissible. 
In general, the jurists define this category of acts, that they call "recommended" or "sunnah," in a number of ways: 
(1) A sunnah act is one whose performance is indicated or encouraged by the law; however, the law falls short declaring it to be obligatory or required.
(2) A sunnah act is one that when performed a person is rewarded for it, but for which he is not punished if he fails to perform it; or, in other words, it is an act that a person is praised for doing and, on the other hand, he is not blamed or censured if he does not perform the act. This is a common definition among the Malikis of North Africa and the Hanbalis.
(3) A sunnah act is an action that one is requested to perform but not in a strict sense. This is a common definition among the Malikis of the Eastern regions (non-North Africans) and among the Shaafi'ees.
(4) A sunnah act is what the Prophet صلى الله عليه وسلم performed on a continual basis, although he would sometimes not perform it with no particular apparent reason for not performing it. This is the definition given by the Hanafis. 
In such definitions, it seems that the jurists were trying to present the most conscise "legalese" definition. As a general concept, one may need to go beyond such a strict legal definition. Perhaps it would be better to consider "recommended" acts as those acts that are becoming of a Muslim and anyone who desires to complete his Islam and his faith would be best served to perform those acts as much as he feasibly can - without them taking on the position of obligatory acts. Furthermore, it should never be forgotten that "sunnah" acts are definitely pleasing to Allaah سبحانه و تعالى and they are a means of getting closer to Allaah سبحانه و تعالى.
There is also another important point to keep in mind concerning the overall category of those acts considered recommended or sunnah. Dhumairiyyah notes,
"Some people are lax with respect to what has been established as sunnah in the usage of the jurists. [They are lax] based on the claim that sunnah acts are those for which one is rewarded for doing them but is not punished for not doing them. But that is in general. At the same time, the scholars have stated, based on numerous ahaadeeth that enourage one to follow and adhere to the sunnah, that one who customarily leaves the sunnah acts is to be punished or castigated. He is doing wrong and committing a sin. The Companions رضي الله عنهم would eagerly perform those acts in a manner similar to how they would perform the obligatory deeds.  al-Laknawi has quoted many texts to that effect in his book Tuhfah al-Akhyaar. As for the jurists' distinction between obligatory and sunnah, that is concerning individual instances and not with respect to leaving sunnah acts completely." 
Another common usage of the word sunnah among the jurists is anything which is juxtaposed with an innovation or heresy (bid'ah). In this sense, the word sunnah may refer to anything that is sanctioned by the sharee'ah. That would include whatever is derived from the Qur'aan, practise of the Prophet صلى الله عليه وسلم, or even the collective action of the Companions.  For example, the jurists may refer to a divorce which is done according to the sunnah (talaaq as-sunnah) vis-a-vis a divorce that is not done in accord with the sunnah (talaaq al-bid'ah). Sometimes, even though the act is not done completely in accord with the sunnah, the act may still have a legal effect and implications but the person has done wrong for performing the act in that fashion. Hence, they make this distinction. 
 The jurists definitely also use the term "sunnah" as a reference to one of the sources of Islamic law. When doing so, though, they are in essence borrowing the legal theorists' definition of the term. The above discussion is concentrating on the their use of the term sunnah as a technical term specific to them as jurists.
 Cf., Abdul Ghani Abdul Khaaliq, Hujjiyah as-Sunnah (Beirut: Daar al-Qur'aan al-Kareem, 1986), pp. 51-68. The most detailed discussion of this topic (recommended acts, the terms used for them and their different implications) available in English is Ahmad Hasan, Principles of Islamic Jurisprudence (Islamabad, Pakistan: Islamic Research Institute, 1993), vol. 1, pp. 78-109.
 Some books try to give an example of this usage in the Prophet's own words صلى الله عليه وسلم. Unfortunately, the hadeeth most often quoted (such as Hasan, p. 82), is, "Allaah has obligated upon you fasting Ramadaan. And I have established (sanantu) for you (as a virtuous act) its night prayers (taraaweeh). Whoever fasts it and prays (those prayers during) it, with faith and hoping for a reward, will have his sins removed from him like the day on which his mother gave birth to him." This hadeeth was recorded by an-Nasaa'ee, Ibn Maajah and others. Unfortunately, this hadeeth is reported through a weak chain and is declared weak by scholars such as Ibn al-Qattan, al-Albani and Shu'ayb al-Arna'oot. 'Abdul Qaadir al-Arna'oot considers it Hasan due to its corroborating evidence; however, he may have been referring only to the concept that whoever prays the nights of Ramadaan shall have his previous sins forgiven, which is recorded in authentic ahaadeeth. Cf., Ali Ibn Muhammad Ibn al-Qattan, Bayaan al-Wahm wal-Eehaam al-Waqi'een fee Kitaab al-Ahkaam (Riyadh: Daar Taibah, 1997), vol. 3, pp. 55-58 and 444-445; Muhammad Naasir ad-Deen al-Albani, Da'eef Sunan an-Nisaa'ee (Beirut: al-Maktab al-Islaami, 1990), p. 76; Shu'ayb al-Arna'oot, et al., footnotes to Ahmad Ibn Hanbal, Musnad al-Imaam Ahmad Ibn Hanbal (Beirut: Muassasat ar-Risaalah, 1997), vol. 3, p. 199 and 217; 'Abdul Qaadir al-Arna'oot, footnotes to al-Mubaarak Ibn al-Atheer, Jaami' al-Usool fee Ahaadeeth ar-Rasool (Maktabah al-Hilwaani, et al., 1972), vol. 9, p. 441.
 Cf., Shawaat, p. 22; Hasan, Principles, vol. 1, pp. 78-80.
 Technically speaking, this is the Hanafi definition for what is known as "non-emphasised sunnah."
 It may be an exaggeration to claim that they performed them in a manner similar to their adherence to the obligatory deeds. Indeed, to treat a recommended deed like an obligatory deed and to insist on it for oneself and others would, in itself, be a type of innovation. The statement in the quote must be understood to mean that they greatly disliked missing the recommended deeds, although they recognised that they were not obligatory.
 'Uthmaan Ibn Jumu'ah Dhumairiyyah, Madkhal li-Diraasah al-'Aqeedah al-Islaamiyyah (Jeddah: Maktabah as-Suwaari, 1993), p. 93. Furthermore, among the sunnah acts, there are some which are described as "emphasised sunnah." For the Hanafis, in particular, an emphasised sunnah takes on almost the same status as an obligatory act. For example, Ibn 'Abideen رحمه الله, a leading Hanafi jurist, wrote, "The strongest opinion is that it is a sin to leave an emphasised sunnah like it is a sin to leave an obligatory act." For more details on this point, see Muhammad Abu al-Fath al-Bayaanooni, al-Hukum at-Takleefi fi ash-Sharee'ah al-Islaamiyyah (Damascus: Daar al-Qalam, 1988), pp. 171-178.
 as-Sibaa'ee, p. 48.
 The Authority and Importance of the Sunnah.