A mentor is someone who has the power to help you reach your dreams. It is the best way for a promising newcomer to advance within the profession, and for the seasoned veteran to leave a legacy. To take that idea one step further, it is my belief that because women are under represented in the sciences, senior women scientists have an obligation to share their experience and knowledge with junior women scientists. A mentor is your own personal teacher, counselor, and guide in your career. Usually it is someone in your field with more experience, who recognizes your potential and is willing to help you achieve it.
Some women never have a mentor, and some have many mentors in the course of their career.
However, mentoring is especially important for women, to help deal with the good ol' boy network or residual prejudice in the workplace. Women can benefit from a mentoring relationship because a mentor can provide feedback on what kind of impression a woman makes on other members of the organization or profession. This is particularly important in the sciences where women are perceived as being outside of the traditional network. Particularly in the early stages of your career, a good mentor is one of the most valuable partners you can have. A mentor can protect you from making the mistakes that he or she made, see that you get the things that he or she never had, and provide constructive criticism.