What is the meaning and difference between CTAF, UNICOM and MULTICOM in the US?
From the Pilot/Controller Glossary


  UNICOM− A nongovernment communication facility which may provide airport information at certain airports. Locations and frequencies of UNICOMs are shown on aeronautical charts and publications.




For example, an FBO may have a unicom frequency that they use to provide information to incoming aircraft and provide traffic advisories (not to be confused with instructions), wind directions, weather, and preferred runways. Unicom frequencies have a ground station at the airport in question.

It is important to note that when talking on a Unicom, the person on the ground that you are talking to in the FBO or airport office is not an air traffic controller.

An example of talking on this would go something like:

Pilot: "KXYZ Unicom, N12345 request traffic advisory"
Unicom: "N12345 No traffic in the pattern, winds XX @ YYY, recommend runway ZZ"
Pilot: "KXYZ Traffic, N12345 will be entering the 45 downwind for runway ZZ"

And so on just like a CTAF if the unicom/CTAF are the same, otherwise switch to the CTAF and make announcements.


  MULTICOM− A mobile service not open to public correspondence used to provide communications essential to conduct the activities being performed by
  or directed from private aircraft.




This is an advisory frequency that does not typically have a ground station associated with it. Pilots are to self-coordinate on the Multicom frequency but there is no ground station at the airport for that frequency. Often many small uncontrolled airports will use the same multicom frequency so you can hear traffic at multiple airports while on a mulitcom.

This is the typical position/intention reports that you normally make at untowered fields, only you are typically just broadcasting your intentions. Sometimes you may have to work with other incoming aircraft such as reporting each other in sight or telling somebody you'll extend downwind to follow them in, etc. 


  COMMON TRAFFIC ADVISORY FREQUENCY (CTAF)− A frequency designed for the purpose of carrying out airport advisory practices while operating to or from an airport without an operating control tower. The CTAF may be a UNICOM, Multicom, FSS, or tower frequency and is identified in appropriate aeronautical publications.


Usually CTAF is used as a multicom, but there are cases where it shares the unicom frequency. If the field is towered and in operation, you'll be working with the tower to coordinate your actions. If the field is non-towered or the tower is not in operation, you'll make position reports and work in the frequency just like a multicom.