I've recommended that this be stickied several times.
Back in the mid-70's, about the time the Mustang II was introduced, Lee Iacocca got his engineers together and gave them an assignment to create a new (for Ford) concept in vehicle production. He wanted to move to "platform" design, where several cars could be built off of one basic pan. At the time Volkswagen was the leader in this concept with their Fox platform, on which they built the Fox, Golf (Rabbit), 100LS and Dasher among others, so the first Ford platform took the name Fox. On it, Ford built millions of Fairmonts, Granadas, Thunderbirds and our beloved Mustangs.
I would highly recommend two books for anyone that is interested in Fox Mustangs: The Official Ford Mustang 5.0: Technical Reference & Performance Handbook : 1979 Through 1993 and Fox-Body Mustang Recognition Guide 1979-1993. Both are excellent sources of true and accurate information about our cars.