The Production Code was a set of rules created in 1922 to ward off governmental censorship, recast in 1929, but ignored, then finally enforced from 1934-1968.

Production Code Timeline (The Code created by Catholics to oversee a Jewish industry for the benefit of Protestants)

1. The Will Hays Era (1922-1945):  In response to political pressure that resulted form a perceived moral decline depicted onscreen & several off screen scandals, Hollywood moguls appointed former Postmaster General to modulate movie content.
2. "Don'ts & "be Carefuls"  (1927-1929):  A list of 11  items that would lead to censorship was published by a committee comprised of studio executives.  The items contained in the list were only marginally adhered to.  Local censor boards edited movies for content, often with city to city specific guidelines, regularly costing the studios millions of dollars annually in damaged & unusable prints.
3. Pre-Code Era (1929-1934):  A Catholic layman (Martin Quigley) & priest (Daniel Lord) created a specific production code that largely reflected Catholic doctrine and beliefs.  Criminal & sexual misconduct must strictly be punished.  This too, was largely ignored, leading to what is commonly called the pre-code era, a 4 year period where violence & sexuality were commonly on display.  What really separates the pre-code era from any other part of the Studio Golden era, however, is the number of strong female characters that took center stage during this time.  Not only were actresses like Joan Crawford, Norma Shearer, Barbara Stanwck & Miriam Hopkins  headlining in strong roles, but Frances Marion led quite a few female writers who churned out content on a regular basis.
4. Code Enforcement (1934-1968):  Forced by threat of a Catholic boycott of movies studio heads appointed Joseph Breen head of the PCA (Production Code Administration) in June of 1934, who ruthlessly enforced the Production Code's guidelines.  Films were issued a PCA certificate of approval that was shown during the opening credits.  Without it films could not be shown in signee theatres (pretty much all theatres).
5. The End of the Code:  The decline of the studio system in the 50's led to the erosion of the code as movies had to complete with television and to a lesser extent foreign films.  In 1968 the rating system in use today (with modifications) was instituted at the behest of Jack Vilenti, the head of the MPAA.