Shirk is when you asocciate other partners and gods with allah the almighty, and when you give some1 allah's attributes.

An example is some people think Jesus is god and they pray to him that is shirk, and idoltry too. And nowadyas some ppl say Muhammad (saws) has allah's attributes and that is shirk too.

A mushirk (one who commits shirk) will never be forgive on the day of judgement unless he repents sincerly because this is one of the biggest major sins and allah gets really angry.