Shaykh Muhammad ibn AbdulWahhab ibn Ali al Yamani al Wasaabi in his book, Al Qawl al Mufeed fee Adillati Tawheed, explains that there are five categories of fear:
1) The first kind of fear is considered worship (ibaadah):
And it is fear from Allah subhanahu wa ta'ala and this kind of worship is a worship of the heart with which we worship Allah.
Allah (swt) says: ((But for him who fears the standing before his Lord, there will be two Gardens.)) [Ar-Rahman: 46]
And Our Creator also says, ((But as for him who feared standing before his Lord, and restrained himself from impure evil desires, and lusts.Verily, Paradise will be his abode.)) [Al Naazi’aat: 40,41]
Allah(swt) also says describing the worship of the angels: ((They fear their Lord above them, and they do what they are commanded.)) [Al-Nahl: 50]
Allah the Most High also says, ((Those who convey the Message of Allah and fear Him, and fear none save Allah. And Sufficient is Allah as a Reckoner.)) [Al Ahzaab:39]
And, ((so fear them not, but fear Me, if you are (true) believers.)) [Aal Emraan: 175]
And, ((... to invoke their Lord in fear and hope..)) [AlSajdah: 16]
And there are more similar ayaat.
The prophet (sala Allahu alayhi wa sallam) also addressed this kind of khawf in many ahadeeth.
In the hadeeth of the seven kinds of people who will be under the shade of Allah, the prophet (sala Allahu alayhi wa sallam) mentioned, “And a man who was called on by a woman of noble status and beauty but instead says, “ Truly, I fear Allah.” [Agreed upon, from the narration of Abu Hurayra-ra-]
Shaddaad ibn Aws (May Allah be pleased with him) narrated that the Messenger of Allah (sala Allahu alayhi wa sallam) said, Allah says, "By my honor and glory, I will not combine two fears upon my slave nor will I combine two securities. If he feels safe from Me in this life,I will make him fear on the Day of Judgment. And if he fears Me in this life, I will give Him safety on the Day of Judgment." [Recorded by Abu Nu'aym, also in Al Saheehah Hadeeth no. 742]
2) The second type of fear is considered SHIRK (associating partners with Allah)
And it is for the servant to fear other than Allah such as fearing the jinn or the dead or other than them by fearing these similar to his fear of Allah, or worse.
Allah subhanahu wa ta’ala says, (( Have you not seen those who were told to hold back their hands (from fighting) and perform As-Salat, and give Zakat, but when the fighting was ordained for them, behold! a section of them fear men as they fear Allah or even more.)) [An-Nisaa’: 77]
And, ((It is only Shaitan (Satan) that suggests to you the fear of his Auliya' [supporters and friends] so fear them not, but fear Me..)) [Aal Emraan: 175]
((Therefore fear not men but fear Me...)) [Al Maedah:44]
3) The third kind of fear (khawf) is a sinful act and disobedience to Allah the Most High.
This is when the servant fears a human being or people and as a result of fearing them leaves a waajib (obligatory act, like shaving the beard, removing the hijaab, not praying at work for fear of being fired) or does a forbidden act due to fear of them while not even reaching the level of being coerced to do the act. This is form of khawf is a sin.
Allah ta’ala says, ((It is only Shaitan (Satan) that suggests to you the fear of his Auliya' [supporters and friends] so fear them not, but fear Me if you are indeed believers.)) [Aal Emraan:175]
And, ((..so fear them not, but fear Me! - And so that I may complete My Blessings on you and that you may be guided.)) [Al Baqarah:150]
And, ((..so fear them not, but fear Me…)) [Al Maedah: 3]
4) The fourth kind of khawf is the natural fear like for the person to fear from the enemy or predatory beasts and snakes, and etc. And this form is permissible as long as the person does not transgress or go beyonds in his/her natural fear.
Allah azza wa jall says concerning His servant and messenger, Musa (alayhis-salaam):
((So he became afraid, looking about in the city..)) [Al Qassas:18]
((So he escaped from there, looking about in a state of fear. He said: "My Lord! Save me from the people who are Zalimun (polytheists and wrong-doers)!")) [Al Qassas:21]
And Musa(as) said, (("My Lord! Verily, I fear that they will belie me..”)) [Al Shu’ara: 12, Al Qassas: 34]
And Musa also said, (("My Lord! I have killed a man among them, and I fear that they will kill me..)) [Al Qassas:33]
And Musa and Haarun (peace be upon them both) said, (( "Our Lord! Verily! We fear lest he should hasten to punish us or lest he should transgress (all bounds against us)."
He (Allah the Most High) said: "Fear not, verily! I am with you both, hearing and seeing.”)) [Taha:45,46]
And the Most Exalted said, ((So Musa (Moses) conceived a fear in himself.We (Allah) said: "Fear not! Surely, you will have the upper hand.”)) [Taha: 67,68]
5) The fifth and final form is the imaginary fear which has no basis or has a weak cause. This is unpraiseworthy and the person who has this fear falls into the characteristic of the cowardly and indeed, the messenger of Allah (sws) sought refuge in Allah from cowardice for it is the manners of the despicable. And for this reason, complete faith, reliance and trust upon Allah and bravery protect from this kind of fear.
-Also see, Shaykh AbdurRahman ibn Naasir As-Sa’dee’s Al Qawl alSadeed fe Maqasid at-Tawheed, pg.116