Summary
What happens in the grave after you die is explained in much detail in the Islamic sources, it is mentioned that in the grave you will be trialled and tested by two angels in regards to your belief, if one passes this test then they will be shown their place in Paradise and given clothes of Paradise and the smell of Paradise will enter their grave which will expand as far as the eye can see. However if one fails this test, then they will be shown their place in the Hell-Fire and given the clothes of the Hell-Fire and the smell of Hell-fire will enter their grave which will constrict until their limbs fold over one another.
The Prophet Muhammad (May the Peace and Blessings of Allah be upon him) said:
“The grave is the first of the stations of the afterlife. So if one is saved from it, then what comes after it is easier than it. And if one is not saved from it, then what comes after it is worse than it.” [At-Tirmidhi: 2308]
Full Answer
Once the soul is taken out of the body at the moment of death, it is then passed through the heavens until the order is given by Allah for it to be placed into the grave where the person is buried.
The Prophet Muhammad (May the Peace and Blessings of Allah be upon him) said:
“The grave is the first of the stations of the afterlife. So if one is saved from it, then what comes after it is easier than it. And if one is not saved from it, then what comes after it is worse than it.” [At-Tirmidhi: 2308]
Muslims believe that after the burial of an individual they will be conscious of their existence in the grave and that they will be questioned by two angels in regards to their belief. This is mentioned in one of the narrations of the Prophet Muhammad (May the Peace and Blessings of Allah be upon him) who said regarding the believer:
Then two [severe] angels shall come and [terrify him and] sit him up next to them and shall ask him: “Who is your Lord?”. He shall reply “My Lord is Allah”. Then they shall ask him: “What is your religion?”. He shall answer them: “My religion is Islam”. Then they shall ask him “Who is this man who was sent among you?”. He will reply “He is the Messenger of Allah (May the Peace and Blessings of Allah be upon him) peace be upon him”. Then they shall ask him “What have you done?”. He shall reply: “I read the book of Allah, then I believed in it and accepted it”. [The angel will terrify him and ask him “who is your Lord?”, “what is your religion?” “who is your prophet?”, and this will be the last trial on earth for the believer, it is then when Allah says : “Allah will establish in strength those who believe with the word that stands firm in this world”, so he will answer my Lord is Allah, my religion is Islam and my Prophet is Mohammad (May the Peace and Blessings of Allah be upon him). _ _[Musnad Ahmad: 18614]
After passing this trial of questioning, the righteous believer is given a reward:
Then a caller will call from the sky: “My slave has spoken the truth, so furnish his grave from Paradise, and clothe him from paradise, and open a door for him from paradise (within his grave)”, so it's goodness and its smell will come unto him, then his grave will be expanded for him as far as he can see.*_ *Then a man will come to him. His face will be handsome, and his clothes will be handsome, and his smell will be sweet. Then he shall say unto him: I bring you glad tidings of that which will make you happy. This is the day that you were promised (46:16). Then he (the believer) will say [and may Allah give you glad tiding] “Who are you?, for your face is the face of someone who comes with good news”. He shall reply: “I am your good deeds, [by Allah, I did not know of you but that you were quick to the obedience of Allah and slow to His disobedience, so may Allah reward you good]”. _[Musnad Ahmad: 18614]
After this, the believer is shown an open gate of Paradise and an open gate of the Hell-fire:
Then a gate of Paradise and a gate of Hell will be opened for him, and it will be said to him: “This is your place if you had disobeyed Allah, but Allah has given you this (Paradise) instead of that”. When he sees what is in Paradise he will say “My Lord bring the hour so that I might return to my family and my wealth”. It will then be said to him: 'be patient'. [Musnad Ahmad: 18614]
The narration then goes on to the describe what will happen to the disbeliever once they are in the grave:
Then his soul will be returned into his body, [he (the prophet) said : verily he will hear the footsteps of his friends who buried him when they leave him]. Then two harsh severe and fearsome angels shall come and [terrify him and] sit him up and shall ask him: “Who is your Lord?”. He shall reply “Huh?, Huh (this is an expression of sorrow), I don't know”. Then they shall ask him: “What is your religion?”. He shall answer them: “Huh?, Huh?, I don't know”. Then they shall ask him “Who is this man who was sent among you?”. He will reply “Huh?, Huh?, I don't know [I heard people saying such-and-such!]”. He will be told thereafter “May you never know!” [Musnad Ahmad: 18614]
The disbeliever is then punished and shown his place in the hell fire:
Then a caller will call from the sky: “He has lied, so furnish his grave from Hell, and open a door for him from the hell fire (within his grave)”, so its heat and hot wind will come unto him, then his grave will be contracted upon him until his limbs are caught up among one another. Then a man will come to him. His face will be ugly, and his clothes will be ugly, and his smell will be vile. Then he shall say unto him: I bring you tidings of that which will harm you. This is the day that you were promised (70:44). Then he will say [and you, may Allah give you bad tidings] “Who are you?, for your face is the face of someone who comes with evil”. He shall reply: “I am your evil deeds [by Allah, I did not know of you but that you were quick to the disobedience of Allah and slow to His obedience, so may Allah reward you with bad, and then will be assigned to him a blind, deaf and mute person who holds in his hand a hammer, if a mountain is hit with it, it would disintegrate, and he will beat him with it until he becomes dust, and then Allah will render him to his initial form, and he will beat him again, and he (the wicked) will cry of sorrow and pain a cry that will be heard by all creatures except humans and jinn, and a door of hell will be opened unto him within his grave and will spread out for him sheets of hell fire]”. Then he shall say: “My Lord do not bring the hour”. [Musnad Ahmad: 18614]
The Prophet Muhammad (May the Peace and Blessings of Allah be upon him) said “When any of you dies, he is shown his position (in the Hereafter) morning and evening. If he is one of the people of Paradise, he is shown the place of the people of Paradise. If he is one of the people of Hell, he is shown the place of people of Hell. He is told, ‘this is your position, until God resurrects you on the Day of Resurrection.” (*_Saheeh Al-Bukhari, Saheeh Muslim_)*
In summary, the grave is the first stage of the afterlife, and it is where you will receive the news of your fate, then comes the next which is the resurrection of the dead in preparation for the Day Of Judgement.
