It's worshipping other than Allah or to associate something else with Him. It's one of the many tricks of Shaytaan to derive you from the straight path.
• There is the minor shirk: What Islam has forbidden because it could get you to major shirk
• And there's major shirk: Giving equals to Allah
Some example of shirk are:
- Imploring the dead (asking them to save us or intercede for us)
- To swear by other than Allah
- To bow down to other than Allah
- To fear the creation more than we fear Allah
- To visit the fortune teller and the astrologer and to believe their lies.
- Wearing amulet or other similar object believing that it will protect us or repel evil.
"Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allah has certainly gone far astray." (4:116)
Shirk is the worst sin, even before the sin of killing someone. Can you imagine?
So if you're out there, living your best life, thinking you're all good because you don't kill anybody, think again. If you do shirk on a daily basis, know that in the eyes of Allah, you are worse than the killer. If you were to die in a state of shirk, Hell would be your resting place.
So, why is Shirk the worse sin? Because it means that you won't follow the words of Allah and the hadiths of the prophet (sallallahu alayhi wa salam).
If you can't even bother to correct this huge sin, what else would you be able to do? Indeed, if you had a good opinion of Allah, you would devote to Him, as He deserves, exclusive worship. It shows the disrespect that one has for the Creator of the universe.
"And [that] He may punish the hypocrite men and hypocrite women, and the polytheist men and polytheist women those who assume about Allah an assumption of evil nature. Upon them is a misfortune of evil nature; and Allah has become angry with them and has cursed them and prepared for them Hell, and evil it is as a destination." (48:6)