Who Are the Khawarij?
Question

How about Khawarij (Kharijites)?
Praise be to Allah.Who are the Khawarij?
The Khawarij (Kharijites) are one of the deviant sects, as is proven by the texts and by scholarly consensus. Al-Bukhari (6934) and Muslim (1068) narrated that Yusayr ibn ‘Amr said: I said to Sahl ibn Hunayf: Did you hear the Prophet (blessings and peace of Allah be upon him) say anything about the Khawarij? He said: I heard him say – and he gestured with his hand towards Iraq –: “From there will emerge people who recite the Quran, but it will not go past their collarbones. They will pass out of Islam as an arrow passes out of the prey.” 
Ibn Majah (173) narrated that Ibn Abi Awfa said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The Khawarij are the dogs of Hell.” (Classed as sahih by al-Albani in Sahih Ibn Majah) 
Are the Khawarij disbelievers?
The Khawarij are followers of whims and desires and innovation who have deviated from the path of Ahl as-Sunnah wa’l-Jama‘ah , but we do not describe them as disbelievers because of their innovation, unlike others who follow whims and desires. 
An-Nawawi (may Allah have mercy on him) said: 
“The view of ash-Shafi'i and the majority of his fellow scholars is that the Khawarij are not to be described as disbelievers; this also applies to the Qadariyyah and the majority of the Mu‘tazilah and other groups that follow whims and desires.” (Sharh Muslim, 7/160)
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: 
The Khawarij who deviated, whom the Prophet (blessings and peace of Allah be upon him) enjoined us to fight, and whom Amir al-Muminin ‘Ali (may Allah be pleased with him), one of the Rightly-Guided Caliphs fought, and whom the leading scholars of Islam among the Sahabah, Taabi‘in and those who came after them were unanimously agreed upon fighting, were not described as disbelievers by ‘Ali ibn Abi Talib, Sa‘d ibn Abi Waqqas and others among the Sahabah; rather they regarded them as Muslims even though they fought them, and ‘Ali did not fight them until they shed blood unlawfully and raided the property of the Muslims; then he fought them in order to ward off their wrongdoing and aggression, not because they were disbelievers. Hence he did not take their womenfolk captive and he did not seize their wealth as booty.” (Majmu‘ al-Fatawa, 3/282)
Ibn ‘Abidin (may Allah have mercy on him) said: 
“Even though you may find that some scholars, within the context of debating with the Mu‘tazilah and other groups, use the word kufr (disbelief) when refuting their arguments, what they meant is that their (the Mu‘tazilah’s) arguments would inevitably constitute disbelief, without implying that they themselves were disbelievers, because what may be concluded from a view is not necessarily to be attributed to the one who holds that view. 
Moreover, they think that they have Shar‘i evidence to support their view, even though they are wrong and are falling into error.” (Hashiyat Ibn ‘Abidin, 3/46) 
Shaykh ‘Abd ar-Rahman ibn Salih al-Mahmud (may Allah preserve him) was asked: Are the Khawarij disbelievers? 
He replied: 
“The scholars differed as to whether they are disbelievers, but the correct view is that they are not to be regarded as disbelievers. ‘Ali ibn Abi Talib (may Allah be pleased with him) was asked about them: Are they disbelievers? He said: They fled from disbelief, but they fell into the innovation of labeling others as disbelievers; we will not fall into the innovation of labeling others as disbelievers and thus label them as such. This is the correct view, in sha Allah, even though their innovations may be described as innovations that constitute kufr.” (Lam‘at al-I‘tiqad, 7/26) 
Innovations of the Khawarij
Among their innovations are the following: 

They think that faith does not increase or decrease .
They think that failing to do something obligatory constitutes disbelief.
They think that committing a major sin constitutes disbelief.
They believe in fighting those Muslims who disagree with them. 

For more about sects, please see this category: Schools of Thought and Sects
And Allah knows best.