Many of the responses here indicate that "true" Islam is a religion of peace.
But, this is wrong for two reasons.
First, Islam is not simply a religion, but both a religion and a political system. While many claim that the religion of Islam is beautiful, the political system is not. Islam's political system seeks to eliminate all other forms of government that oppose Allah's absolute rule.
Abu Ala Maududi, a leading Islamic scholar, in his book, “Jihad in the Cause of Allah” indicated:
“Islam is not merely a religious creed or compound name for a few forms of worship, but a comprehensive system which envisages to annihilate all tyrannical and evil systems in the world and enforces its own program of reform which it deems best for the well being of mankind.
“It must be evident to you from this description that the objective of Islamic jihad is to eliminate the rule of an un-Islamic system and establish in its stead an Islamic system of state rule. Islam does not intend to confine this revolution to a single state or a few countries: the aim of Islam is to bring about a universal revolution.”
It would behoove us to never speak about Islam as merely a religion again. Rather, consider saying that "Islam is a comprehensive religious and political system that strives for universal domination."
Second, while it is true that Islam seeks "peace," the definition of peace used by Muslims differs fundamentally from how non-Mulslims define peace.
Most non-Muslims would define peace as being able to live in a community, tolerating differences, in a state of harmony characterized by the lack of violence.
The Qur'an, however, tells us a condition required to usher in peace with its lack of fighting and violence.
Muhammad's recitation in Sura 8:39 commands Muslims: "So fight [jihad] them until there is no more Fitnah (disbelief [non-Muslims]) and all submit to the religion of Allah alone (in the whole world)."
The cessation of violence and fighting will occur only when people universally submit to Allah's laws, that is, becomes Muslim.
Further, jihad is prescribed or required of all Muslims. Muhammad commanded all to fight in Suah 2:216 "Fighting [jihad] is prescribed for you, and ye dislike it. But it is possible that ye dislike a thing which is good for you, and that ye love a thing which is bad for you. But Allah knoweth, and ye know not."
Jihad, fighting in the cause of Allah, is not simply violence. It involves all activities that advance Islam. These include debating, dialoguing, preaching, writing, publishing, immigration into non-Muslim countries, financial activities, and self-sacrifice (including suicide missions).
The fact that 99.99% of Muslims are peaceful does not mean that Muslims are not engaging in jihad to promote Islam. Most support jihad financially. Only a few Muslims need to engage in violent jihad at any one time.
The men who flew planes into buildings on 9/11 were terrorists. The day before, on 9/10, they were simply peaceful Muslims practicing Islam, the religion of peace.
Tamerlan and Dzhokhar Tsarnaev were nice kids on or before April 14th. But, on April 15th, 2013 they became terrorists when their pressure cooker bombs exploded during the Boston Marathon.
Nidal Malik Hasan was just an ordinary U.S. Army major and psychiatrist on November 4, 2009. But on the 5th he shouted "Allahu Akbar!" as he killed 13 people and shot and injured another 31. He considered himself a Soldier of Allah and regarded himself as a Mujahideen waging "jihad" against the United States.
These and many other Muslims were living peacefully among us for years, practicing the religion of peace until the time they became terrorists.
Islam, inherently, is not simply a religion but also a political system. Islam does not promote tolerance and harmonious multiculturalism. Rather, Islam seeks the total destruction of non-Islamic cultures and total obedience to the laws of Allah.