What does SOS and MAYDAY mean? : Origin and use

What does SOS mean? What does MAYDAY mean? What is the difference? PAN PAN?
SOS first.
An SOS was an international distress signal transmitted by Morse code. With the advent of long distance radio transmissions in the early 1900’s it was possible to communicate with radio using Morse code signals. (Samuel Morse and Alfred Vail developed the electric telegraph during the 1800’s, more detail). During the early 1900’s the Germans introduced a radio distress signal based on this technology that was adopted worldwide a few years later.
This signal consisted of three dots, three dashes and three dots again, “…_ _ _…”
These signals represent S O S in Morse code, but it would appear as though the letters were irrelevant, it had all to do with the distinctive sound. Three shorts, three longs and three shorts repeated over and over again were distinctive and easily recognizable. (Transmitted with no spaces between the three letters).
After the fact meanings were given to SOS such as Save our Souls, Save our Seamen, Save our Ship, Survivors on Shore…. endless possibilities. (One list I found had more than 100, from the serious to the ridiculous).
The process of giving names after the fact is called a bacronym, which was a new concept to me. (I felt better when the spell-check was also caught out.)
With the development of new technology, satellites and so forth, Morse code has been replaced and no longer used. It is still used as a visual distress signal, as in one could place objects on the ground to form an SOS, to get attention from the air. No matter how you do the SOS it will be recognized, even if seen upside down.
Originally Mayday is to aircraft as SOS was to ships. The main difference being that the distress signal was “voiced” or “spoken”. Origin around 1923 by Fred Mockford who was an English radio officer given the task to think of a word that could be easily understood as a distress call. From the French “Venez m’aider” meaning “come help me".
Then there is PAN PAN which is also a distress signal, but not for life threatening circumstances.
The protocol of using theses signals is interesting and lots of interesting detail to be found at Wikipedia for a better understanding
http://en.wikipedia.org/wiki/SOS
http://en.wikipedia.org/wiki/MAYDAY
Image from Wikipedia