Channels are able to turn down their own rational mind, ego, logical processor and personal belief system to allow spiritual/psychic/intuitive messages to flow through them, just as if they were a telephone wire. These messages usually come from a defined and known source.
Channel work can be very difficult for most humans because we all have very strong personal beliefs, ideals, logic and dependence on our rational minds. If any of these things turn up while channeling, purity is lost and messages become filtered by the channel’s own paradigm. Think of channeling as a message going from one telephone to another and the personal paradigm of the channel as static. It becomes challenging to get a clear and accurate message.
Channels tend to be a conduit for complete forces. That means, they channel the soul, the Universe, the Divine, specific vibratory forces like love, healing, etc. Many channels are both channels and mediums and excel at both when they are well experienced and dedicated to the art.