Self-esteem, is the opinions, feelings and thoughts you have about yourself. Although childhood experiences play a large role, self-esteem can evolve and change in reaction to life events and experiences.
People with poor self-esteem determine their worth by how they are currently performing. They often put their efforts into maintaining a public "persona" (Carl Jung). Healthy self-esteem is based on our ability to see ourselves accurately and accept and value ourselves unconditionally. We all have strengths and limitations and people with good self esteem and confidence can acknowledge their strengths and limitations and work towards self improvement unhindered.