RSS is the original XML format that really opened up syndication online.  There were many attempts before and none really caught on until Really Simple Syndication did so.  There is a lesson there that the formats and languages that are the most simple usually spread rapidly.  RSS is based on XML that although is a bit railed on these days for being overly bloated, it was largely the hero of freeing data into services and promoting the web service api nature of the web today.  JSON has since probably become a better data format but RSS is still very viable like ATOM in syndication formats due to their simplicity.