Led by angry and politically savvy Washington wives (and mothers), the Parents Music Resource Center quickly gained the attention of US Senate, leading to a 1985 investigation into "pornographic content" in rock music. As a result, the Recording Industry Association of America (RIAA) introduced a labeling system that still stands today. As for what gets labeled - well, that's left up to the record companies, so standards arguably vary. The RIAA is a bit 'late to the party' - the rating system developed by the Motion Picture Association of America (MPAA) has been in place since 1968, with a few revisions and changes along the way.
After the PRMC incident, things were quiet until 1996, when the Telecommunications Act was passed. The Telecommunications Act required the creation of a rating system for television compatible with "V-chip" blocking technology, and mandated V-chip technology for all new television sets manufactured after a certain date.
Also in 1996, Microsoft, SurfWatch, CompuServe and Cyber Patrol announced they would implement the Recreational Software Advisory Council Internet (RSACi) rating system, allowing parents to block access to Web material without this rating.
The RSACi was adopted into the Internet Content Rating Association (ICRA) just three short years later. The ICRA believes "that self-regulation leads to the best balance between the free flow of digital content and protecting children from potentially harmful material". In this case, it's still about 'thinking of the children'. They're not alone in these thoughts, with at least sixteen other groups working on a similar mission: making the Internet a safer place for children.