A Medium is a person that can hear people that have passed on. This ability is very simular to the childhood game of telephone. The medium repeats what they hear or see from the spirit.
This of coarse is dificult in that your dealing with the spirits trying to communicate, and the medium trying to understand. Unfortunatly many messages can be misunderstood.
On a bright note about this - with practice actuate messages can be relayed. This of coarse depends upon the spirit, and the medium.
The best advice I can offer on this skill, is getting a mentor to better assist you in this area.
Something else to check out! This site also has links to some famous gifted web sites.