A Medium is a human being who is a phychic and has the ability to contact spirits when ever they feel like it. Be aware that not all phychics are mediums.
Mediums are also "channerlers"
A medium gets information from the spirit world by several means, Most will use the following:
Clairvoyance: It Means clear seeing, it is refered to mediums who see spirits. they may see them mentally or in a physical from.
Clairaudience: This one means clear hearing, you will hear spirits physically or mentally like i said above.
Clairsentience: This one means you can sence when a spirit is near by.