This world is the place of trials and tribulations
Question

What is this world?
Praise be to Allah.This world is the place of striving and the Hereafter is the place of reward or punishment, where the believers will be rewarded with Paradise and the disbelievers will be punished with Hell. 
Paradise is good and none but those who were good will enter it. Allah is Good and accepts nothing but that which is good. So the way of Allah with His slaves is to test them with calamities and tribulations, so that the believer may be known from the kaafir and so that the truthful may be distinguished from the liar, as Allah says (interpretation of the meaning): 
“Do people think that they will be left alone because they say: ‘We believe,’ and will not be tested.
And We indeed tested those who were before them. And Allah will certainly make (it) known (the truth of) those who are true, and will certainly make (it) known (the falsehood of) those who are liars, (although Allah knows all that before putting them to test)” [al-‘Ankaboot 29:2-3] 
Victory and success cannot be achieved except after tests which will bring the good forth from the evil and tell the believer apart from the kaafir, as Allah says (interpretation of the meaning): 
“Allah will not leave the believers in the state in which you are now, until He distinguishes the wicked from the good. Nor will Allah disclose to you the secrets of the Ghayb (Unseen)” [Aal ‘Imraan 3:179] 
Among the trials with which Allah tests His slaves in order to distinguish the believers from the disbelievers is that which He mentions in the aayah (interpretation of the meaning): 
“And certainly, We shall test you with something of fear, hunger, loss of wealth, lives and fruits, but give glad tidings to As-Saabiroon (the patient).
Who, when afflicted with calamity, say: ‘Truly, to Allah we belong and truly, to Him we shall return.’
They are those on whom are the Salawaat (i.e. who are blessed and will be forgiven) from their Lord, and (they are those who) receive His Mercy, and it is they who are the guided ones” [al-Baqarah 2:155-157] 
So Allah tests His slaves, and He loves those who are patient, and gives them the glad tidings of Paradise. 
Allah tests His slaves with jihaad, as He says (interpretation of the meaning): 
“Do you think that you will enter Paradise before Allah tests those of you who fought (in His Cause) and (also) tests those who are As-Saabiroon (the patient)?” [Aal ‘Imraan 3:142] 
Wealth and children are a trial by means of which Allah tests His slaves, to know who will give thanks for them, and who will be distracted from Allah by them: 
“And know that your possessions and your children are but a trial and that surely, with Allah is a mighty reward”
[al-Anfaal 8:28 – interpretation of the meaning] 
Allah tests us, sometimes with calamities and sometimes with blessings, to show who will be thankful and who will be ungrateful, and who will obey and who will disobey, then He will reward or punish them on the Day of Resurrection: 
“and We shall make a trial of you with evil and with good. And to Us you will be returned”
[al-Anbiya’ 21:35 – interpretation of the meaning] 
Testing is according to one’s faith; the most severely tested among mankind are the Prophets, then the next best and the next best. The Prophet (peace and blessings of Allah be upon him) said: “When I fall ill, my pain is equivalent to the pain of two men among you.” (Narrated by al-Bukhari, 5648). 
Allah tests His slaves with different kinds of trials. 
Sometimes He tests them with calamities and tribulations to distinguish the believer from the disbeliever, the obedient from the disobedient, the grateful from the ungrateful. 
Sometimes Allah tests His slaves with calamities; when they commit sin, He punishes them with calamities so that they might come back to him, as He says (interpretation of the meaning): 
“And whatever of misfortune befalls you, it is because of what your hands have earned. And He pardons much” [al-Shoora 42:30] 
“And indeed We seized them with punishment, but they humbled not themselves to their Lord, nor did they invoke (Allah) with submission to Him” [al-Mu’minoon 23:76] 
Allah is merciful to His slaves; He sends repeated tribulations upon the ummah so that they may return and repent to Him, and give up that which Allah has forbidden, and so that Allah might forgive them. Allah says (interpretation of the meaning): 
“See they not that they are put in trial once or twice every year (with different kinds of calamities, disease, famine)? Yet, they turn not in repentance, nor do they learn a lesson (from it)” [al-Tawbah 9:126] 
It is part of the mercy of Allah that disasters befall sinners in this world, so that their souls might be purified and they might come back to Allah before they die: 
“And verily, We will make them taste of the near torment (i.e. the torment in the life of this world, i.e. disasters, calamities) prior to the supreme torment (in the Hereafter), in order that they may (repent and) return (i.e. accept Islam)”
[al-Sajdah 32:21 – interpretation of the meaning] 
Sometimes Allah tests His slaves with calamities in order to raise them in status and to expiate for their sins, as the Prophet (peace and blessings of Allah be upon him) said: “No misfortune or disease befalls a Muslim, no worry or grief or harm or distress – not even a thorn that pricks him – but Allah will expiate for some of his sins because of that.” (Agreed upon. Narrated by al-Bukhari, 5641)