Only Muslims make it into Heaven and then everyone else who is excused by Allah for various reasons, such as being mentally ill, a newborn baby, being completely unaware of Islam, etc.
Muslim means, "one who submits". Allah has defined what Islam is. There are five pillars: Declaration of faith in One God only, observing the five daily prayers, performance of Hajj, performance of fasting during the month of Ramadan and paying of Zakat.
Ultimately, anyone who says they are Muslim are to be considered Muslim in this life. It is ultimately up to Allah who is truly a Muslim or not. By definition, it means one who satisfies these five conditions is a Muslim. However, it is up to Allah to decide who was really not able to satisfy these conditions at any point in their lives and decide if they go to Heaven or not. No one in this life can judge that for a person.
The relation we have to Allah is that of a slave and Him being our Master. So we must live our lives in obedience to Him, otherwise we are rejecting our purpose in life. We are made to recognize Him (51:56), thus prayers are absolutely necessary.
Also, I must add that to all the people of the past religions, their past Prophets told them to accept the future Prophets which come to you. It is incumbent that you give you allegiance to them and obey them. This is a covenant from Allah to you, and the Qur'an is a reminder of that. (Hence, why the Qur'an constantly calls itself a reminder, because it reminds everyone of their covenant to Allah). Therefore, rejecting this covenant from your own Prophet is like rejecting your Prophet. So not only is the rejection of that future Prophet wrong, but by rejecting him, you rejected the covenant you made with your own Prophet.
Whether you are aware of that or not is between you and Allah. But, this Prophet which all the past nations with Prophets must accept is the Holy Prophet Muhammad (peace and blessings of Allah be upon him). It is obligatory on every person of a faith from a previous Prophet to accept him.
All this was to outline in clarity why people of different faiths will not necessarily be accepted by Allah for their faith. If they knowingly reject the Last Prophet of Allah, then their faith will not be accepted, because Islam came with all the past Prophets, their followers were Muslims, now they can only remain Muslim by accepting the final Prophet of Allah, Muhammad (saw). One of the conditions of being Muslim in the six articles of Faith, which is to believe in the Oneness of God, His Angels, His Prophets, His Books, His Divine Decree and the Day of Judgment, requires this.
Now, if the people of all other faiths believe in One God, the Day of Reckoning (however form they understand it to be in), His Divine Decree (that He has the final say in everything), His Angels, they pay their own form of Zakat, etc. They are still required to remain so by accepting the final Prophet of Allah, so that they accept Allah's final law, in the Qur'an, and thus accept the performance of Hajj to Mecca instead and pray five times daily which is the best way to keep away from all filth and sin. On top of that, they will become humble by paying Zakat often and performing fasting during Ramadan. These pillars and articles of faith are designed for mankind to reach the pinnacle of their closeness to Allah. No other religion offers this.
As for Heaven and Hell, it now remains to say why all of this is important with regards to it. For each of these things, such as fasting, prayers, paying charity, etc., there are doors for each one. And whoever performs those deeds will be able to go through them for doing them correctly and as they should be done. Some will go through all of them. The pleasures for such a Paradise are very high.
Anyone who refrains from paying Zakat will find that their wealth will come to them in the form of a bald-headed snake on the Day of Judgment, torturing them and saying to them, "I am your wealth."
Those who fail to perform their prayers all five times daily, will have to make up for it on their balances on the Day of Judgment, otherwise they will have to use their other good deeds to make up for it.
The unforgivable sin in Islam is Shirk. Shirk will be punished for on the Day of Judgment. For a Muslim, Hell cannot touch them whoever had tears during prayers, whose forehead was touching the ground during prostration very often or anyone who frees a male slave, or two female slaves (which equals the freeing of one slave). For every bone in the body of a male slave who is freed ransoms every bone in the body of a Muslim, every two bones of a female slave ransoms one bone for the person who frees her (which is why two female slaves must be freed).
The fire will not touch one whose feet are covered in the dust for which they are covered with from fighting for the sake of Allah in Jihad. And those who die in Jihad and are martyred are raised in the same state they died in on the Day of Judgment. They will smell like musk on that day.
As for the trial of the grave, there will be questions asked from the person such as what religion they are of (Islam is the correct answer), which Prophet did they follow (the answer should be Prophet Muhammad (saw)), etc. Thus, if one answers otherwise, then they will be punished in the grave until the Day of Judgment.
As for Hell, Muslims will be taken out of Hell and everyone else who had absolutely no inclination of obedience to Allah in them, then they will be left to burn in Hell. These will be the absolute worst people in Hell. Some scholars have the opinion that despite this, eventually everyone will leave Hell as it is only a temporary abode for everyone, though for a very long time. Most believe that they will burn in Hell eternally, and they will never come out.
This is not to say there would not be Muslims in Hell for a short time. They would be interceded for by the Holy Prophet (saw), then the Prophets, then the Angels, then the Muslims, then finally, Allah will take a Handful of people out of Hell.
This in short, is the description of Heaven and Hell in summary, though to be honest, there is a lot to read about the descriptions of the Day of Judgment and the pleasures of Paradise and the characteristics of Hell.
