Not all Muslim is a terrorist, but almost every terrorist is a Muslim.
Oftentimes, when people say this, they say it because they define any violent act committed by someone from a Muslim family to be an aaspect of some "global jihad" while they are more than willing to dismiss the thousands of murders committed in America by Christians as isolated events or merely criminal acts.
Regardless, it's hard to look at the IRA, ETA, LTTE, KKK, Shining Path, FARC, UNITA, etc and say that "almost every terrorist is a Muslim".