Who is Sunni Muslim?
The Muslim who follows Sunnah.

What is sunnah?
The teaching and actions or examples of Muhammad, Prophet of Islam (peace be upon him). Personally, I don't like to label myself as Sunni Muslim as there is no such word "Sunni Muslim" in Quran and Hadith. 

The Sunni Muslim believe that Muhammad, Prophet of Islam (peace be upon him) didn't nominate a successor before his departure from the world. Hence, the most of his prominent companions (may Allah be pleased with them) gathered and elected Abo Baker Siddique, (May Allah be pleased with him)  the closest friend, and father in law as the first caliph through democratic way.   

There are four schools of thoughts in jurisdiction of Sunni Muslims such as Hanafi, Shafi, Maliki, Hunbali. They all have slight differences in their method of practicing Islam, however the destination of all is the same i.e Allah and the road to the destination is Prophet (peace be upon him) . The first four caliphs who were differed from one another in nature, wealth, and attitude, but were the most beloved companions of Prophet (peace be upon him). The similarity among all companions was Taqwa (God-fearing).

Now come to the point I like you to know. I came across a debate whether the prophet of Islam, Muhammad (peace be upon him) is alive or death.
Among all schools of thoughts there is no discrepancy that Prophet Muhammad (peace be upon him) tested the death. Allah says in Quran, [39:30]
"Indeed you (O dear Prophet Mohammed – peace and blessings be upon him) will taste death, and they (the disbelievers) too will die.". As a matter of fact, all living things including martyrs about whom Allah forbade us to call "death" in Quran [2:154]  too have to test the death according to this verse of Quran [03:185] "Every soul must taste death".

There is almost all Sunni Muslims who believe that the death is a mean of transferring soul  from temporary  life to perpetual life. As per this concept, the soul of pious people resting peacefully in their grave till the resurrection day, whereas, the soul of transgressors are lying unrest in their grave.

Majority Muslims, as per this concept, believe that the Prophet, Muhammad (peace be upon him) resting in his grave with sound and safe. I was astonished when I came to know that a group of Muslims denied this fact by referring to the verse of Quran as cited above [39:30] and putting aside the authentic Hadith, which states that the Prophet, Muhammad (peace be upon him) while replying to a query of Sahabah (may Allah be pleased with them) said, "Allah has prohibited the earth from consuming the bodies of the Prophets." [Abu Dawud Book 002, Hadith Number 1526.]

Furthermore, there is another authentic Hadith Narated By Abu Hurayrah: The Prophet (صلى الله عليه وسلم) said: If any one of you greets me, Allah returns my soul to me and I respond to the greeting. [Abu Dawud Book 004, Hadith Number 2036:]

In response to the last Hadith as referred above, I would like to refer to Surat Al-Ahzab verse No 56.

"Indeed Allah and His angels send blessings on the Prophet; O People who Believe! Send blessings and abundant salutations upon him." (Everlasting peace and unlimited blessings be upon the Holy Prophet Mohammed.)

Note: There is no need to explain how much blessings and abundant salutations are sent upon beloved Prophet (peace be upon him)during 24 hours.As per this Hadith, He (peace be upon him) responds to them all.  Can we imagine how many times a day the soul of Prophet is returned to him in order to respond to all the greeting?  Absolutely NO! Besides, Allah says in  Surah No 21 and verse No 107 " And We have not sen t you O Muhammad (peace be upon him) except as Mercy to the worlds". He has been sent as Marcy to the worlds till doomsday.  It means there is no more  Marcy to the world as soon as he (peace be upon him) was died? Allah blessed us wisdom to carry out His orders by understanding the priorities or scale of preference rather than drawing the stereotypical image hidden in words, but the whole content testifies that He is alive in his grave. 

Despite having these kind of differences which also based on Taqwa (God-fearing), we are united as Muslims under the shade of the word of purity " there is no worthy of worship except Allah and Muhammad (peace be upon him) is His last Messenger." In Allah`s eyes, it is a crime to be involved in sectarianism. Allah says in the Quran (3:103). "Hold on firmly together to the rope of God, and be not divided among yourselves"

The above references are too obvious to argue, yet different opinions but in a respectful way are welcomed.