Channelling is a process by which information is obtained from sources other than the medium or channel themselves. It involves psychic ability which is usually developed over a period of lifetimes. Some people with this gift may have other intuitive abilities also.
Normally the person will enter a trance state, which can be very light or very deep. Most channels eventually train themselves to go under. In this state of heightened awareness, the medium picks up information from other entities. This is translated into words and gestures.
Some channels specialize in just one or two main guides or entities which they use repeatedly and are comfortable with. For example Jane Roberts' Seth was her main entity which she channelled. In this case through Jane, Seth created several books on psychic phenomena and the nature of reality. She also, through Seth, led many group classes on individual psychic development.
Some channels offer spiritual counseling through the use of such personal sessions. In this setting, the medium will access either his or her own personal guides or higher beings, or will reach those which are connected to the client. All kinds of beings can be channelled, from earth devas to your pet cat, extra-terrestrials, angelic beings, Mother Mary, even rivers, streams, trees, and dead relatives can be accessed, depending upon the range of the channel.
How does this work, exactly? There is a portion of the brain known as the pineal gland, this is the doorway through which unlimited psychic regions are reached. Many channels will tell you that they have a sensation of pressure or heaviness in their head as they are going in to trance. This is their signal that they are "hooking up" with the entities who wish to speak.
Surprisingly, on occasion a channel will access an entity who is non-verbal. In this case, frequently an "interpreter" is found in spirit who can assist in helping speak for the non-verbal energy.
No two channelling sessions are ever the same, as each individual has their own issues and questions. No query is off-limits for the guides, however it helps to prepare a list beforehand of what areas you want to cover. People may have relationship issues, past life questions, health problems, or career questions. Frequently they want to speak to someone who has passed on.
Channelling is very exciting, it is a beautiful and unique art form, and very helpful for finding information that may not be obtainable in any other way.