Sometimes people get stuck trying to answer this question. As soon as we begin to talk about sin, we can sound judgmental.
The truth is that we can't really know God without understanding sin, since sin has distorted our world from the perfection God intended. Sin has distanced us from God, putting a barrier between us.
But we also need to remember that sin is not about judgment or blame. As with all of our questions about God and faith, this question needs to be answered with love and grace, because that's how God treats all of us sinners—with love and grace.
Sin is anything that is contrary to the will of God. It can be stubbornness, rebellion, or outright disobedience. Sin is the cause of all pain, hurt, confusion, and doubt in the world, and it is not part of what God intended.
Genesis tells us that humans were created in the image of God and that we were created to reflect his glory. Romans 3:23 reminds us that "All have sinned and fall short of the glory of God." The glory of the Lord, which we were intended to reflect and experience, is now dimmed because of the entrance of sin into the world.
However, God has not just left us in our sinful state. Because God loves us, he has provided us a way out of this life of sin, a way to be in relationship with him as he originally intended.