Ideally, a friend is someone you trust who shares a deep level of understanding and communication with you. A good friend will show a genuine interest in what's going on in your life, what you have to say, and how you think and feel about things. He or she will accept you for who you are and listen to you attentively without judging you, telling your how to think or feel, or trying to change the subject.
As friendship works both ways, a friend is also someone you feel comfortable supporting and accepting, and someone with whom you share a bond of trust and loyalty. A good friend will feel comfortable sharing things about themselves with you.