SdntPilot was close. What he was describing is known as "CTAF" or the Common Traffic Advisory Frequency. Unicom is a service that's usually provided by an FBO on the field. The purpose for Unicom is to give airport advisories (active runway, wind direction/speed, barometric pressure, etc.), radio checks, and for ordering fuel. At uncontrolled fields with a Unicom, the CTAF and Unicom frequencies are the same. The only difference that a pilot announcing his position calls to "Traffic" and the pilot calling Unicom addresses it to unicom. Examples (I use my home field of PVU):
"Provo Traffic, Cessna 106ES turning a left final, touch and go, Provo."
"Provo Unicom, Cessna 106ES, we are inbound for landing, can we get some gas?"
If the airport is unattended, or there is no Unicom, the CTAF is 122.9 and called "Multicom." Usually, if there is a Unicom, CTAF and Unicom will be 122.8. There are some exceptions to that. If there is an FSS on the field, Unicom will be 122.8, and CTAF will be 123.6. If you are flying at an airport with a tower, but the tower has closed for the evening, then CTAF will be the tower frequency, and Unicom will be as noted by the aeronautical chart, flight guide, etc - but still usually 122.8.
Anyway, that's how I understand it. Hope that helps. Let me know if you have any questions.
Jack @ AUS