The topic of study for the scholars of hadeeth is everything that has been narrated or reported concerning the Prophet صلى الله عليه وسلم. They desire to gather all of that information to determine what of it is sound and acceptable and to distinguish that from what has been narrated through unstrustworthy or unacceptable means.
Hence, first and foremost, they concentrate on anything narrated about or from the Prophet صلى الله عليه وسلم. All of the information about the Prophet صلى الله عليه وسلم that one could imagine was passed on and recorded in detail, the extent of which is not comparable to anyone in the history of mankind.
The purpose of study or goal of the scholars of hadeeth has greatly affected their technical definition of the word "sunnah." Indeed, their definition of the word sunnah is the broadest of all definitions, trying to encompass everything that was passed on concerning the Prophet صلى الله عليه وسلم. In general, the definition of "sunnah" from the scholars of hadeeth perspective is: "What has been passed down from the Prophet صلى الله عليه وسلم of his statements, actions, tacit approvals, manners, physical characteristics or biography, regardless of whether it was before he was sent as a prophet or afterwards." 
To make this definition clearer, examples of each category shall be given below:
An example of one of his statements is the hadeeth narrated by Sayyiduna 'Umar Ibn al-Khattaab رضي الله عنه that the Messenger of Allaah صلى الله عليه وسلم said, "Surely, all actions are but driven by intentions and, verily, every man shall have but that which he intended. Thus, he whose migration was for Allaah and His Messenger, [then] his migration was for Allaah and His Messenger; and he whose migration was to achieve some worldly benefit or to take a woman in marriage, his migration was for that which he migrated." (Recorde by al-Bukhari and Muslim)
An example of one of his actions is the following report recorded on the authority of Ummul Mu'mineen 'Aa'ishah رضي الله عنها: "When he [the Prophet صلى الله عليه وسلم] would perform the two [non-obligatory] rak'atayn of Fajr in his house, he would then lie on his right side." 
An example of the Prophet's صلى الله عليه وسلم tacit approval is found in the hadeeth from Saheeh Muslim in which Sayyiduna Anas رضي الله عنه was asked if the Prophet صلى الله عليه وسلم ever performed the two rak'atayn of prayer before the Maghrib (Sunset) Prayer. He replied, "He used to see us performing them and he would neither order us to do them or forbid us from performing them."
An example of a report describing the Prophet's صلى الله عليه وسلم manner or behaviour is: "The Prophet صلى الله عليه وسلم was more bashful than the virgin girls kept in their private compartments. If he saw anything he did not like, we would recognise it by [the expression on] his face." (Recorded by al-Bukhari and Muslim)
Reports describing the Prophet's صلى الله عليه وسلم physical characteristics are numerous. There are reports about his height and physical appearance, for example, Sayyiduna Ibn 'Umar رضي الله عنهما narrated that the Messenger of Allaah صلى الله عليه وسلم had approximately twenty grey hairs. 
Another example is the report from Sayyiduna Jaabir Ibn Samurah رضي الله عنه that the Prophet صلى الله عليه وسلم had a full beard. (Recorded by Muslim)
Reports concerning his biography include the report that he married Khadeejah when he was twenty-five years old while they were living in Makkah and before he had received his first revelation. Also included in this category would be the reports that he lived in Makkah for thirteen years after receiving revelation and then migrated to Madeenah.
Again, these types of reports all fall under the hadeeth specialists' definition of the word "sunnah," even though some of them may have no direct bearing on Islamic law. For the hadeeth specialists they are important because those scholars are concerned with everything that has been narrated from or about the Prophet (peace and blessings be upon him). 
 as-Salafi, p. 15; Shawaat, p. 20
 With this wording, this is recorded by at-Tirmidhi. According to Shaykh al-Albani رحمه الله, it is saheeh. See Muhammad Naasiruddeen al-Albani, Saheeh Sunan at-Tirmidhi (Riyadh: Maktabah at-Tarbiyyah li-Arabi li-Duwal al-Khaleej, 1988), vol. 1, p. 132
 Recorded by at-Tirmidhi in ash-Shamaa'il. According to Shaykh al-Albani رحمه الله, this report is saheeh. See Muhammad Naasiruddeen al-Albani, Saheeh al-Jaami' as-Sagheer (Beirut: al-Maktab al-Islaami, 1986), vol. 2, p. 873. These kinds of reports demonstrate the amount of information that has been preserved and passed on concerning the final prophet and messenger of Allaah (peace and blessings be upon him).
 The Authority And Importance Of The Sunnah