The Arabic word "Islam" (إسلام) means surrender or submission.
Allah (God) says in the Qur'an in chapter 3 verse 19:
"إِنَّ الدِّينَ عِندَ اللَّهِ الْإِسْلَامُ ۗ وَمَا اخْتَلَفَ الَّذِينَ أُوتُوا الْكِتَابَ إِلَّا مِن بَعْدِ مَا جَاءَهُمُ الْعِلْمُ بَغْيًا بَيْنَهُمْ ۗ وَمَن يَكْفُرْ بِآيَاتِ اللَّهِ فَإِنَّ اللَّهَ سَرِيعُ الْحِسَابِ"
"Indeed, the religion in the sight of Allah is Islam. And those who were given the Scripture did not differ except after knowledge had come to them - out of jealous animosity between themselves. And whoever disbelieves in the verses of Allah, then indeed, Allah is swift in [taking] account."
The word Islam is also linked to Salam (سلام) which means peace, and is the greeting used by Muslims all around the world and even the Prophets of previous nations.
Islam is built upon 5 pillars:
As Muslims we believe that all the previous Prophets and Messengers have called to Islam (to submit to the creator and not associate any partners with him) and that innovation (in religion) and change overtime is what has lead human kind to divert from pure monotheism.
In conclusion, Islam is submitting to the will of God in everything we do, starting with the belief that there is one creator who is self sufficient, he neither begets nor is begotten and nor is there to him any equivalent.