I respectfully disagree there (partly), Andreas:
Charter airlines have very much in common with low cost airlines. They are airlines which cater for the needs of similar customer groups, namely people on a low budget flying for pleasure. Their strategy is to give these people access to air travel by offering prices below the normal level. They do so by maintaining a quite low cost structure achieved by similar means as the low costs.
One could indeed call the charter airlines (which exist as a phenomenon since the late 60s) the forefathers and frontrunners of today´s low cost airlines. The latter of course refined the recipe be even more consequent cost cutting (hence attracting even more customer groups) and other destinations.
The boom of charter airlines came with the advent of two socio-economic phenomena in the 60s:
1) bigger airplanes (especially the invention of the widebody slightly later) created a greater supply of transportation capacity, making it effectively cheaper (basic laws of supply and demand)
2) people, especially in Germany and the UK, started travelling. After the war, there were several phases of very high demand for certain goods and services in Germany, labelled "waves": the eating wave, then the clothes and the furniture wave, and finally, in the 60s with their climax in the late 60s/early 70s, the travel wave. It ebbed with the mid/late 70s´ energy crises.
Hope this explains some things, Kay.