this is my opinion only....
i believe a medium is a person who acts like the middle man so to speak in the comminication from you to spirit..
the easiest way to describe this is to use an example of something simple and known to all..a telephone
you have two people wanting to communicate..you have the tool to do so ie..the telephone itself..but to connect the two you need the middle man in this case the phone line....
the phone line acts as the receptor..without this you can not connect...
this is the same scenario for a medium..you have a person here on the earth plane and that in spirit..the medium can be the inbetween connection for the messages etc to be crossed over..
a medium is also what some people know as "sensitive to spirit"..although i believe everyone has this ability at some part of their lives...sensitives /mediums have mastered this ability to use it for the communication in spirit for the betterment of others..most mediums are quite advanced in their abilities and have good control over the spirit communication they are receiving..