In the simplest terms, the noun medium is a go-between.
In art, it's a material used to create, like oil paint or clay. In science, it's a substance through which something is transmitted, like air. In communications, it's a means of sharing information, like the Internet or TV. Medium can even refer to people who purportedly communicate with the dead; they're a go-between connecting the living and the spirit worlds.
The word medium has two plural forms—mediums and media—and which one you use depends on what you're talking about.
If your subject is art, it's okay to use mediums or media. For science and communication, media is generally preferred. And if you're talking about fortunetellers and psychics, you always refer to them as mediums.
(Aren't you glad we started with the simple version?)