The most common use of the word heart is the Hebrew word Leb (pronounced layb). It is used widely for the feelings, the will and even the intellect; the center of anything.
See it in some verses. Here is the first time it is written: in Genesis 6:5-6. Then the Lord saw that the wickedness of man was great on the earth, and that every intent of the thoughts of his heart was only evil continually. The Lord was sorry that He had made man on the earth, and He was grieved in His heart. We are certainly made in God's image, we even have the same "heart". Only man's heart is corrupted by sin; God's heart is perfect.
A story about a man's heart that I find especially interesting is in Exodus. It starts in chapter 7 and verse 3 But I will harden Pharaoh’s heart that I may multiply My signs and My wonders in the land of Egypt. Since mankind has a free will and the means to make his or her own decisions, then why would God do this to the heart of Pharaoh? It sounds like Pharaoh didn't have a will in this situation. Like he was a puppet being made to do that which God wanted. This isn't the case. God knew the heart of Pharaoh. He knew his will and intellect. He knew what was in the center of Pharaoh's intentions. Pharaoh was going to be a key player in God's means of doing His work in Egypt and freeing His people.
Here's where it becomes interesting. When Moses spoke to Pharaoh, it was God speaking to him according to Exodus 7:1. The words from God, the demands of God, and the acts of God were causing Pharaoh to resist that which God wanted from him. Hardened is defined as: to strengthen, prevail, harden, be strong, become strong, be courageous, be firm, grow firm, be resolute, be sore. Pharaoh made a choice, God hardened his heart. Or shall we say it another way - Pharaoh's heart was hardened because God's presence invaded his way of life.
Our feelings, will and intellect are such important parts of our life - this is our center. We have been given a free will that can decide as to what we are going to do with God. Sometimes He makes a move in our life or needs to bring about a change. Sometimes His ways are not what we want to have happen, or make us have to get out of our place of comfort. What will you do with God? Will you be open to Him, or will He harden you?