When we think of idols, we think of physical figures mankind calls "gods", or uses in worship of anyone but God, but we seldom think of other things which can be considered idols.
In our lives, there are a number of things which draw our hearts away from God. These things may not even necessarily be bad things. Your job, your friends, your hobby, your family, romance, money, food or drink, none of these things are inherently evil, and if used correctly, can be used to glorify God. But they can also have the adverse effect. They can cause us to neglect, or even oppose God.
Imagine you have a job in the financial department of a large corporation. You have been given a budget, and have been commanded to use it to further the company's aims. You even have written instructions from the CEO himself on how you may and may not spend your budget. Now imagine the janitor asks for a large portion of that budget to be spent almost entirely on him.
In this scenario, the janitor is supposed to be in the building, and he performs a function for the company. But obviously, the CEO won't be happy if you're giving all that money to the janitor.
In the same way, money isn't a bad thing. In fact, it's a rather useful thing. Used correctly, it can glorify God. But some people take that to an extreme. To these people, they're only really "happy" when they have enough money, and of course they never do have enough money. This causes them to obey their janitor rather than their CEO.
Safe to say, terrible things have been done for the sake of money. It is not the only idol we set up in our hearts. Many things take the place of our CEO when they should only really be the janitor. Still others ought to be the rats that scavenge through our bins, yet they are also given a strong influence over how we spend our spiritual budget.
Think of the most important things in your life. Are you using these things to worship God, or are they things you worship instead of God? Are you willing to sin to increase these things? If so, they are an idol.
In Ezekiel 14:4, God told Ezekiel that whoever comes to inquire of him, the Lord would answer that person according to the multitude of his idols. God does not take kindly to being considered just one more brick in a perfectly stable wall. He is the rock upon which our lives should be built, the cornerstone which the builders rejected, the strong tower in whom we should take refuge. We are serving the same God as Ezekiel. When you come to God with idols in your heart, God takes those into account. So think again of those things that are important in your life. If you can honestly say you at least try to put God ahead of them, you do well. But if they are so vital to you that you would even sin for them, that's a golden calf you need to throw in the fire.