Hypocrisy is to make apparent goodness and to conceal evil. And it is of two types:

 

Hypocrisy is to make apparent Islaam upon the tongue and to conceal kufr in the heart.
Explanation of Shaikh al-Fawzaan:
It-haaf al-Qaaree of Shaikh al-Fawzaan:

Hypocrisy is to make apparent goodness and to conceal evil. And it is of two types:

1. An-Nifaaq al-I’tiqaadee (Hypocrisy in Belief)

This is Major Disbelief and the outright hypocrite is worse than an outright unbeliever, who makes apparent that he is not a Muslim. That is because it is known about the outright disbeliever that he is a disbeliever, and that he has enmity – however, the hypocrite deceives the Muslims. He makes manifest that he is from the Muslims whereas in reality he is an enemy to them. He makes apparent that he is a Muslim, whereas [inwardly] he is a disbeliever.

يُخَادِعُونَ اللَّهَ وَالَّذِينَ آمَنُوا وَمَا يَخْدَعُونَ إِلَّا أَنفُسَهُمْ وَمَا يَشْعُرُونَ

“They [think to] deceive Allah and those who believe, but they deceive not except themselves yet they perceive it not.” Baqarah 2:9
For this reason Allaah placed them in the lowest depth of the Hellfire, beneath the worshippers of the idols and the disbelievers, because they are worse than the disbelievers, and He says about them:

هُمُ الْعَدُوُّ فَاحْذَرْهُمْ ۚ قَاتَلَهُمُ اللَّهُ ۖ أَنَّىٰ يُؤْفَكُونَ

“They are the enemy, so beware of them. May Allah destroy them; how are they deluded?” al-Munaafiqoon: 4
Hypocrisy in Belief cannot co-exist with Imaan (faith) ever.

2. The Second category is an-Nifaaq al-‘Amalee (Hypocrisy in Deeds)

This is where a a person is a believer both outwardly and inwardly, however there emanate from him some of the traits of the hypocrites that reduce his Eemaan and he is threatened with severe punishment, but he does not exit the Religion. So this is termed as an-Nifaaq al-‘Amalee or as an-Nifaaq al-Asghar. Concerning this, there is the statement of the Messenger:

أَرْبَعٌ مَنْ كُنَّ فِيهِ كَانَ مُنَافِقًا خَالِصًا : إِذَا حَدَّثَ كَذَبَ , وَإِذَا وَعَدَ أَخْلَفَ , وَإِذَا عَاهَدَ غَدَرَ , وَإِذَا خَاصَمَ فَجَرَ ، فَمَنْ كَانَتْ فِيهِ خَصْلَةٌ مِنْهُنَّ كَانَتْ فِيهِ خَصْلَةٌ مِنَ النِّفَاقِ حَتَّى يَدَعَهَا

“There are four traits when found in a person, they render him a sheer hypocrite. And whoever possesses one of them, he possesses a trait of hypocrisy until he gives it up: If he speaks, he lies. If he makes a promise, he breaks it. If he is given a trust, he proves treacherous, and if he argues, he becomes abusive.” [1]
[1] Reported by al-Bukhaaree no. 34, Muslim no. 58. Hadeeth narrated by ‘Abdullaah b. ‘Amr b. al-Aas (radiyallaahu ‘anhumaa)

So there emanates from this believer hypocrisy in deeds from time to time. That is a deficiency in his Eemaan – and he is deserving of the threat of punishment, however he does not exit from the Religion because of it.

And this hypocrisy is also ar-Riyaa (showing off with deeds of piety) which the Messenger (salallaahu a’alaihi wassallam) feared for his Companions and he called it Minor Shirk (ash-Shirk al-Asghar). He said:

“That which I fear for you the most is Minor Shirk” They asked: “And what is Minor Shirk, O Allaah’s Messenger?” He responded: “Ar-Riyaa (showing off with deeds of piety). Allaah will say on the Day of Resurrection when the people will be recompensed for their deeds: “Go to those for whom you used to show-off you deeds or piety in the world. See if you find with them any reward.” [1]
[1] Ahmad in Al-Musnad (5/428), Al-Bayhaqee in Shu’b al-Eemaan (5/333), Al-Baghawee in Sharhus-Sunnah (14/323-324) from Mahmood b. Labeed (radiyallaahu ‘anhu). Al-Mundhiree said in At-Targheeb wat-Tarheeb, “Isnaaduhu jayyid (its chain is good)” (1/69).

The Messenger (salallaahu ‘alaihi wassallam) also said:

“Shall I not inform you regarding that which I myself fear for you even more than the Dajjaal?” They said: “Yes, of course.” He said: “Ash-Shirk al-Khafee (the Hidden Shirk). A man stands to pray and he beautifies his prayer when he sees a man looking towards him.” [1]
[1] Ahmad in Al-Musnad (3/30), Ibn Maajah in As-Sunan (no. 4204), At-Tahaawee in Sharh Mushkil al-Aathaar (no.1781)

When he prays amongst the people, he beautifies his prayer, but when he prays in his home of in a hidden location, he hastily performs the prayer. This is what the Sahaabah would fear for themselves with a strong fear. And there is not a single person whose soul is free from this affair, so a person should be fearful of it. It is for this reason they (Salaf) would say:

“No one fears it except a believer, and no one feels safe from it except a hypocrite.”
So a Muslims fears for himself from this hypocrisy, and that is the Minor Hypocrisy.