It all started back in 1964 when Al Jaffe and the gang at MAD noticed that other big magazines (Playboy, Life, etc) were doing fold-outs (centerfolds). In typical MAD style, they decide to do a spoof and make a fold in instead. The rest is history.
Al himself has personally created very single one of them since and is still doing them today at age 87! That's incredible.
Here's a really good NY Times article on Jaffe that I really enjoyed.
Meanwhile check out the AMAZING collection of interactive fold ins past and present (LINK).

