Question on the meaning of ‘Islam’

It has been popularly propagated that Islam means peace for all. But does it true? The question was triggered after the Sunni Islamic organization ISIS took the responsibility for a blast that took place inside the masjid of Shia Muslims in Pakistan, a Muslim dominated country. Although both the sects follow Islam, they have differences which raised the question of what is the real meaning of Islam? Let us see the differences which led to terrorist attacks and violence in which Muslims were killed under Islamic rule.