Shirk in Islam -associating a partner with Allah- is proven by the Shahada, the First Pillar of Sunni faith, wherein a person can't even be considered a Muslim if he does not profess that Mohammad is Allah's Messenger. This is also known as idolatry.<quoted text>
The best definition of Shirk, comes from Christianity, which makes the false claim that Jesus was the son of God and the absurd claim that Jesus was God. Christianity is a religion based upon Shirk.
To us, Muhammad, Jesus and Moses were men, who were obedient SLAVES of God.
"Ashhadu an la ilaha illa 'llah; ashhadu anna Muhammadan rasulu 'llah"
"I witness that there is no god but Allah, and Muhammad is the messenger of Allah."