The Q Codes were developed many years ago for use in CW (Morse) communications. They were used to abbreviate commonly used expressions thus reducing the number of letters that were transmitted. They have persevered over the years, and several have become regular expressions in ham radio, such as QSL, QSO, QRP, etc.
There are many more than illustrated in the following graphic, but these are the most widely used.
For a much more extensive list:
last modified: 2007-11-13 15:19:14