Idol worship is praising any form of idol. The most well known form is worshipping a statue resembling some form of god, however putting anything before your could be consider idol worship. Idol worship is considered a sin by the ten commandments.
Throughout the old testament of the bible there are many stories of nasty things happening to people who commit acts of idol worship so if you are of the Jewish or Christian faith it is probably not a good idea, however it is easy to argue that both of these religions do practice idol worshipping (the cross etc.) Other religions such as Hinduism and Buddhism actively embrace idol worship.
The reason for so much resentment towards idol worship may stem from the story of Abraham, the alleged father of monotheism. The story goes that Abraham was the son of Terah, a proffesional idol maker.
One day Abraham said to his mother "The idols are hungry, you must feed them." So his mother cooked a meal for the idols and left it in the room with them, but they did not eat it. So Abraham said to his mother, "That food was not good enough for them, you must do better." So his mother made a delicious banquet for the idols and again left it in the room with them. At some point Terah went out for a while. When he returned he found that all that all the idols had been smashed except for a large one wielding a hammer. Terah then screamed at Abraham, asking him why we he smashed the idols. To this Abraham replied, "They were hungry. I just fed them. When the little ones reached to take their portion, the big idol killed the rest of them." Terah then said, "What nonsense! I made these idols. They cannot move. You smashed them Abram! You put the hammer in the hand of the remaining idol. Now you tell me lies and expect me to believe that the idol did this terrible thing." Abraham answered this by saying, "You are right. You made these idols with your own hands. They cannot move, they cannot breathe, they cannot deliver you from your enemies, and they cannot protect you from sickness or death. Why do you worship them? There is only one true God. He created the heavens and the earth. He is the only God that I will worship!” With that, Abram smashed the last idol and left town on his journey of faith.