Dual Band, Tri-Band and Quad Band are relates to GSM network. A band is a GSM frequency that the cellphone/camera works on. The cellular frequencies are the sets of frequency ranges within the ultra high frequency band that have been allocated for cellular phone use. GSM network operates in four different frequency ranges (850/900/1800/1900).
For instance a cellphone/camera that works on the 850 and 1900 would be a Dual Band. If it works on all 4 GSM bands is a Quad Band. HCO 2G is Quad Band, which means it works on 850/900/1800/1900. HCO 3G is Dual Band, which works only on 850 and 1900.
AT&T uses 850/1900 for its 2G service and the same frequency for its 3G service. T-Mobile uses 850/1900 for its 2G service and 1700/2100 for its 3G service. This means the HCO 3G camera works on AT&T network as a 3G device but 2G device on T-Mobile network.