Low Self Esteem is when a person has a very low opinion of themselves. They often feel they are not entitled to good things, or other peoples attention and time. Often expectations of themselves will be very high to the point of being un reachable by anyone's standard.
People suffering with Low Self-Esteem are often highly self critical, and suffer high levels of shame.
The negative beliefs about the self in people with low self esteem usually have their roots in past experience. They are conclusions that have been drawn on the basis of these experiences. For example; If a person is told as a child that they will never amount to anything, the chances are they will have taken this information as absolute truth as children tend to believe what adults say. This is then likely to have become an integral belief within that persons belief system that they are no good. Part of therapy will involve understanding where these beliefs come from and learning ways to challenge and change these beliefs.