Richard,

Code groups are random groups of letters, numbers, or a combinations of 
both. Each group usually, but not always, contains five characters.

The set of records that I learned CW from in 1968 used code groups, 
rather than words, for their lessons. Initially, at least, that made it 
hard to second guess which letter was coming next. Even now, copying 
random code groups will separate the men from the boys, especially at 
higher speeds.

The intruder that I mentioned was transmitting CW, but there have been 
similar stations that transmitted using either SSB or AM. Most of the 
voice transmissions were number groups, but I have encountered a few 
letter group voice transmitions as well.

If you are interested in these transmissions, take a look at 
www.numbers-stations.com.

-- 
Mike Duke, K5XU