Channeling and Astral Travel are related disciplines, but rather different in many ways.
When Channeling, the person opens their self up and allows a "spiritual entity" (pick your description) to use their body and/or voice as a vehicle for communication. A Channeler might transmit the communication either by voice, writing, or some similar way. (Some artists are Channelers, for example. )The person doing the channeling is often in some degree of trance, although this isn't strictly necessary. Often, there needs to be some degree of "surrendering control" by the Channeler, so that the Spirit may speak or act freely enough to communicate. In this sense, it is similar to "possession" by a spirit, although the channeling arrangement is benign and usually mutually beneficial.
Astral Travel also happens in a trance state, although a "spiritual entity" does not come to visit. Rather, the Spirit of the Traveler leaves the physical body and journeys in the World of Spirit. Communication with the Spirit Worlds happens here, the difference being that one can communicate and interact with several Spirits at the same time when you go to them. When Channeling, you can only talk to the one who comes to visit.
Astral Travel is a skill that is developed by Shamanic practice. In Astral Traveling, the spirit leaves the physical World of Form and journeys to the World of Spirit to gain information. The traditional realm of the Shaman, Astral Traveling or Journeying is achieved by suspending normal consciousness.
Sleep is one such method of reaching the Astral, but not the preferred method due to the lack of control most people experience in the dream state. (The exception is those who manage to master the technique of Lucid Dreaming, but this is difficult for most.) More commonly, people reach the Astral planes via various meditation and trance techniques. Occasionally, consciousness is changed by use of mind-altering chemicals, although this method isn't recommended unless under the direct supervision of an experienced practitioner.
~Flame RavenHawk