Shirk is the opposite of Tawheed. It is attributing a partner to Allah in His Lordship or in His Worship or in His Names and Attributes.
Contrary to popular belief, shirk does not just consist of believing that there is another god besides Allah or by prostrating to an idol. There are many more ways that shirk can be committed.
As Allah has promised us that shirk is not forgiven (if one were to die without repenting from it), it is mandatory for every Muslim to study the different forms of shirk so that they can avoid it and also deepen their understanding of tawheed.
Allah has refuted the followers of shirk by giving parables to explain the futility of their actions.
Some of them are:
“Allah puts forth a similitude: a (slave) man belonging to many partners (like those who worship others along with Allah) disputing with one another, and a (slave) man belonging entirely to one master, (like those who worship Allah Alone). Are those two equal in comparison? All the praises and thanks be to Allah! But most of them know not.” [Surah Az-Zumar (39) :29]
“And whoever assigns partners to Allah, it is as if he had fallen from the sky, and the birds had snatched him, or the wind had thrown him to a far off place.” [Surah Al-Hajj (22) :31]
“O mankind! A similitude has been coined, so listen to it (carefully): Verily! Those on whom you call besides Allah, cannot create (even) a fly, even though they combine together for the purpose. And if the fly snatched away a thing from them, they would have no power to release it from the fly. So weak are (both) the seeker and the sought.” [Surah Al-Hajj (22) :73]
“He sets forth for you a parable from your own selves, – Do you have partners among those whom your right hands possess (i.e your slaves) to share as equals in the wealth We have bestowed on you? Whom you fear as you fear each other? Thus do We explain the signs in detail to a people who have sense.” [Surah Ar-Rum (30) :28]
Shaikh Saalih Al-Fawzan explains these parables.