ARccOS stands for Advanced Regional Copy Control Operating Solution. It is a copy protection technology developed by Sony for DVD-Video content. Since, Content Scrambling System (CSS), the primary copy protection used for retail DVDs, can be easily circumvented, several solutions were developed to discourage copying by modifying the DVD-Video compilation itself in specific ways or the disc to hinder ripping attempts.
ARccOS sets a target in DVD copying/ripping software by inserting intentionally corrupt data into sectors on a DVD disc. This can cause DVD copying/ripping software to produce errors or imperfect results. Since DVD ripping software usually tries to copy every sector on the disc with no guidelines, ARccOS can be very successful at stopping copying attempts.
However, ARccOS can reportedly be beaten by use of commercial software such as AnyDVD or freeware software such as DVDFab Decrypter. DVD playback problems generally don't occur due to ARccOS since a DVD player can be instructed by the DVD media on what to read, and what to ignore.
While Sony reportedly discontinued ARccOS, some DVDs continue to use its extra protection.