Mentoring is a system of semi-structured guidance whereby one person shares their knowledge, skills and experience to assist others to progress in their own lives and careers. Mentors need to be readily accessible and prepared to offer help as the need arises - within agreed bounds.
Mentors very often have their own mentors, and in turn their mentees might wish to ‘put something back’ and become mentors themselves - it's a chain for ‘passing on’ good practice so that the benefits can be widely spread.
Mentoring can be a short-term arrangement until the original reason for the partnership is fulfilled (or ceases), or it can last many years.
Mentoring is more than ‘giving advice’, or passing on what your experience was in a particular area or situation. It's about motivating and empowering the other person to identify their own issues and goals, and helping them to find ways of resolving or reaching them - not by doing it for them, or expecting them to ‘do it the way I did it’, but by understanding and respecting different ways of working.