A medium is someone who communicates with spirits. Or more accurately, someone with whom the spirits find it easy to communicate.
Our loved ones who have deceased are always around us. Sometimes they try to get a message across to us.
Sometimes the message is important -- other times they just want to remind us that they still think we're buffoons with control issues.
Usually we can not hear them. Depending on your loved ones, this may be a blessing!
A Medium is a person who can hear, or see, or feel spirits of people or animals that are on the other side of the transition we call death.
The word 'medium' is used in a very mechanical way, in much the same way that television and radio are each considered a medium.
A radio or television carries messages we can not see or hear, and converts them to a form we can see or hear. A Medium carries a message from one plane to another.
How is a Medium different from a Channel?
Channelling is a form of psychic functioning where a person (the Channel) allows a spirit to enter their physical body and manipulate it. The most common form of channelling is done by speaking through the Channel's voicebox.
To the lay person it looks like the theatrical representations of possession, but it is not the same.
Generally, to be a good Channel a person will devote many years developing their skills in connecting and communicating with the 'incoming entity'.
No, I do not work as a Channel. I worked as a Channel a few times in my youth, and found it to be physically exhausting and, in my personal experience --no reflection on channelling-- a very crude method compared to the other ways of communicating available to me.