CDMA 850 is CDMA technology operating in the Cellular (850 MHz) frequency band.
Before the existence of GSM 850, the Cellular band was often referred to as the "800 MHz" band. "850 MHz" implies a different frequency band, but the two are often used interchangeably. "800 MHz" and "850 MHz" usually refer to the exact same frequency band. 850 is technically a more accurate description of the frequency range, although "800" is still common.
However, Sprint Nextel has started offering CDMA service in the ESMR band, which is also near 800 MHz. It is important not to confuse the two bands.
See: CDMA 800
See: Cellular
See: ESMR
In specs, Phone Scoop refers to the ESMR band as 800, and the Cellular band as 850.