When I Googled "identity crisis" this is the site I found to have the most succinct definition: http://psychology.about.com/od/theoriesofpersonality/a/identitycrisis.htm. It summarizes Erik Erikson's theory or personality development. He believed that an identity crisis was an integral part of human development and maturation. We ALL face identity confusion and crisis at some point or multiple times during our lives.
What I found most interesting in this link was the statement "...the balance between identity and confusion lies in making a commitment to an identity." Meaning, it is important to actually decide on one, whether we choose it consciously or not. Some people move into their core identity more easily it seems, while others struggle and fight to find it. But the act of committing to one is, I believe, an essential step toward finding contentment and confidence in yourself.