A transmission media is the medium over which information travels from the sender to receiver. In other words a communication channel is also called as a medium. Different media have different properties and used in different environments for different purposes. The purpose of physical layer is to transport a raw bit stream from one computer to another.
Media are roughly grouped into two classes:

  1. Guided media
  2. Unguided media

Guided media: Guided media is a communication medium which allows the data to get guided along it. For this the medium need to have a point to point physical connection.

Unguided media: The wireless media is also called as unguided media.

The examples of guided media are copper wires and fiber optics. Similarly radio and lasers through the air are examples of unguided media.
In computer networking, the medium affects nearly every aspect of communication. Most important, it determines how quickly and to whom a computer can talk and how expensive the process is.