Its usage in the Sharee'ah: "the guidance
which the Messenger and his
companions were upon in terms of knowledge, belief, speech and action.
This is the Sunnah, the following of which is obligatory, whose adherents
have been praised, and whose abandoners have been rebuked .
The term 'sunnah ' is also used for the various acts of worship and beliefs,
just as everything which is contrary to it is termed 'bid'ah' (innovation)"
Its usage in the Sharee'ah: "They are
the salaf  of this Ummah, from among the Sahaabas
(companions) and the Taabi'een (successors of the Companions) and whoever
follows them in goodness till the Day of Judgement. They are those who
unite themselves upon the Book and the Sunnah and upon their leaders (i.e.
their scholars) and those who travel upon that which the Messenger ,
His Companions and those who followed them in goodness were upon". 
So Ahl us-Sunnah wal-Jamaa'ah have been described with sticking to the Sunnah and [in all circumstances] avoiding/disregarding the invented matters and innovations in the Deen (religion).
By the word Jamaa'ah the totality of
muslims is not meant, nor those who are largest in number or the great
majority . This is because the Messenger
mentioned that the
Taa'ifat ul-Mansoorah 
is a single group from among the seventy three groups.
As-Salaf as-Saalih  and Ahl ul-AtharThe phrase 'As-Salaf as-Saalih' (The Righteous Predecessors) is synonymous with Ahl us-Sunnah. The Ahl us-Sunnah has also been named 'Ahl ul-Athar' (The People of Narration) meaning: Followers of the Sunnah transmitted from the Messenger and his companions.
Ahl ul-HadeethThey have also been named 'Ahl ul-Hadeeth (the People of Hadeeth) as they take from the Sunnah of the Messenger that which has been reported, and what is contained therein of knowledge/cognizance, and are the followers of his guidance outwardly and inwardly. The Ahl us-Sunnah, therefore are the Ahl ul-Hadeeth with respect to this meaning.
The naming of the Ahl us-Sunnah, At-Taaifatul Mansoorah, and Al-Firqat un-Naajiyyah with Ahl ul-Hadeeth is a matter which is elaborate and extensive in the view of the Salaf of this Ummah because it is a requirement of the various texts, of the description of the actual state of affairs and of factual evidence. This has been established from Ibn Al-Mubaarak, Ibn Madeenee, Ahmad ibn Hanbal, Imaam Bukhaaree, Ahmad ibn Sinaan and others besides them (radiallaahu-anhum) .
Many of the scholars have named them (the Ahl us-Sunnah) in a similar manner and have introduced their books, compositions and compilations with this name. For example the book: [Aqeedat us-Salaf Ashaab ul-Hadeeth] (The Aqeedah of the Salaf, the Companions of Hadeeth) of Imaam Isma'eel Saaboonee 449 H.
Also see Majmoo ul-Fataawaa of Shaikh ul-Islaam Ibn Taymiyyah 4/9, 95 where he has named the Ahl us-Sunnah with Ahl ul-Hadeeth.
Al-Firqat un-NaajiyyahAnd also: the 'Firqat un-Naajiyah (The Saved Sect) ': this is the one which is rescued from the Fire due to its adherence to the Sunnah of the Messenger . This (term) has been taken from the hadeeth: "And this Ummah will split into seventy-three sects, seventy-two will be in the Fire and one in Paradise and that is the Jamaa'ah" 
At-Taa'ifat ul-MansoorahAnd this is why many amongst the Salaf and the Scholars of the Deen name the Ahl us-Sunnah with 'Al-Firqat un-Naajiyyah' and those who are clearly upon the Truth with 'At-Taa'ifat ul-Mansoorah' (the Aided Group) . These are the ones whom the Messenger himself has named due to his saying: "There will never cease to be a small group (Taai'fah) from my Ummah clearly upon the Truth until the Hour is established"
Al-Jamaa'ahThey are also called 'The Jamaa'ah' and sometimes 'Ahl ul-Jamaa'ah'as has preceded.
The (word) Jamaa'ah, which is the Jamaa'ah of the Ahl us-Sunnah, those who come together and unite themselves upon the Truth, is taken from the word Ijtimaa' (an assembly, gathering) and that is the opposite of Furqa (separatedness, disunity). The word Jamaa'ah contains the meaning of 'Ijmaa' which is 'Ittifaaq' (unanimous agreement) the opposite of which is 'Ikhtilaaf' (disagreement). The Ahl us-Sunnah have been described as those who have united themselves upon the Usool ud-Deen , have agreed upon them and have united upon the scholars of the Deen.
Ahl ul-Ittibaa'They have also been described with 'Ahl ul-Ittibaa'' (The People of Adherence and Imitation) because it is from their way: to follow the Aathaar (tracks) of the Messenger outwardly and inwardly, to follow the path of the first and foremost of this Ummah from among the Muhaajireen and the Ansaar  and to follow the advice of the Messenger when he said: "You must follow my Sunnah and the Sunnah of the Rightly Guided Caliphs after me. Hold onto it (my Sunnah) and bite onto it with your molars. Beware of the newly invented matters for every bid'ah is misguidance... "
1 See [Mukhtaar us-Suhhaah] p. 317 and Ibn Mandhoor's [Lisaan ul-Arab] 13/220 -228
2 See [Al-Wasiyyat ul-Kubraa Fee Aqeedatu Ahl us-Sunnah wal-Jamaa'ah] p23, [Sharh Aqeedat ul-Waasitiyyah] by Muhammad Khaleel Kharraas p.16, and [Sharh Aqeedat ut-Tahaawiyyah] p.33.
3 See [Al-Amr bil-Ma'roof wan-Nahee anil-Munkar] by Ibn Taymiyah p.77.
4 See [Lisaan ul-Arab] 8/53-60.
5 Salaf : Its meaning in the arabic language is 'those who precede, have gone before'. Its usage: a word used by the earliest scholars for the first three generations of muslims and those who are upon their way in accordance with the hadeeth of the Messenger which is reported by Bukhaaree: "The best of generations is my generation, then those that follow them, then those that follow them". Imaam Abu Haneefah (rahimahullaah) d. 769 (150 H.) said: "Adhere to the athar (narration) and the tareeqah (way) of the Salaf (Pious Predecessors) and beware of newly invented matters for all of it is innovation" [Reported by As-Suyootee in Sawn al Mantaq wal-Kalaam p.32] By clinging to their way, holding on to their beliefs and understanding them as they did, worshipping Allaah in His Oneness, upon the Authentic Sunnah of the Messenger in the manner of the Companions one is guaranteed success in this life and security from the Fire in the next life. When questioned by his companions about those who will be saved from the Fire, the Messenger replied: "They are those who are upon what I and my companions are upon" [Reported by Tirmidhee from Amr ibn al-Aas - Hadeeth Hasan] All the great scholars from the earliest to the later times have advised clinging to the way and methodology (manhaj) of the Salaf and adherence to it as it is the only means of deliverance. Ibn Taymiyyah (rahimahullaah) says: "There is no fault/criticism for the one who manifests/proclaims the way (madhdhab) of the Salaf, who attaches himself to it and refers to it. Rather, it is obligatory to accept that from him by unanimous agreement (Ittifaaq) because the way (madhdhab) of the Salaf is nothing but the Truth (Haqq)" [Majmoo al-Fataawaa 4:149]
6 See [Al-I'tisaam] by Ash-Shaatibee 1/23 and [Sharh Aqeedat il-Waasitiyyah] by Haras p.16-17 and [Sharh Aqeedat it-Tahaawiyyah] p.33
7 See [[Sharh ul-Aqeedat il-Tahaawiyyah] by Abi Izz al-Hanafee p.330 and [Risaa'il fil-Aqeedah] p.53.
8 The term Bid'ah is meant for every belief, speech, action, which is not from the Messenger and His companions and has no precedence from them. This term is relevant in every age and era and will remain so until the Day of Judgement. Allaah has provided every generation with trustworthy ones who will preserve His Deen
9 Abu Umaamah reports that the Prophet "My Ummah will split up into seventy-three sects, seventy-two in the Fire and one in Paradise" We said. Describe them to us. He said : "As-Suwaad al-A'dham (the Great Majority)" Reported by al-Laalikaa'ee in [Sharh Usool il-I'tiqaad] and Ibn Abee Aasim in [as-Sunnah]. The Great Majority here does not refer to the great majority of people in every age and era. Rather this is specific for the era of the Companions and the Taabi'een. Abdullaah Ibn Mas'ood said: "The Jamaa'ah is whatever (agrees) with the Truth. Even if it is only one person" . Ishaaq ibn Raahawaayah (d.238) said: "If you were to ask the ignorant people about the Great Majority they would say: The majority of people. They do not know that al-Jamaa'ah is the Scholar who clings to the narrations from the Prophet and his way. So whoever is with him (the scholar) and follows him, then he is al-Jamaa'ah" Reported by Abu Nu'aym in [Hilyat ul-Awliyaah]. In the era of the Companions and their Followers the Taabi'een the Great Majority in that time were upon the Truth. This was due to the fact that people were in nearness to the time of the Messenger and the Messenger attested to the credibility of the best of generations. As for those who came after them then the fact that they are many is not to be considered due to the generality of the texts which give evidence that evil will increase and spread amongst the people, the Ummah will split into seventy-three sects and that Islaam will return as something strange.
10 The Aided Group: a synonymous term for Ahl us-Sunnah wal-Jamaa'ah.
11 Reported by Ibn Abee Aasim in [Al-Kitaab was-Sunnah] (1/33). Al-Albaanee said: Hadeeth Saheeh.
12 The addition 'As-Saalih' means 'righteous'. See footnote 5 which has preceded for an explanation of this point.
13 Ibn Al Mubaarak (rahimahullaah) said: "According to me, they are the Ashaab ul-hadeeth" Imaam Bukhaaree said: Ibn al-Madini said: "They are the Ashaab ul-Hadeeth (People of Hadeeth)" Ahmad ibn Hanbal said: "If the Aided Group is not the Ashaab ul-Hadeeth then I have no idea who it is!" Shaikh Abdul Qaadir al-Jilaani said: "As for the Saved Sect it is the Ahl us-Sunnah wal-Jamaa'ah and there is no name for the Ahl us-Sunnah except one and that is the Ashaab ul-Hadeeth"
14 Reported by Abu Daawood in his [Sunan] in the 'Book of Sunnah' no. 4597
15 Reported by Muslim, Tirmidhee, Ibn Maajah and Al-Haakim - Saheeh
16 Usool ud-Deen: The Fundamentals of the Religion. This refers to the matters of belief (Aqeedah), including the manner of belief in Allaah Azzawajall and all that is due to him from the meanings of Tawheed regarding His Essence, His Names and His Attributes
17 Muhaajireen: Those who emigrated from Makkah to Medinah with the Messenger . Ansaar. Those who received the emigrants in Medinah and aided them.
18 Reported by Ibn Aasim in his [Book of Sunnah] Al-Albaanee said: Its isnaad is Saheeh and its narrators, all of them, are trustworthy.