RSS is based on XML, a widely used standard for textual information exchange between applications on the Internet. RSS feeds can be viewed as plain text files, but they’re really designed for computer-to-computer communication.
RSS is actually just one standard for expressing feeds as XML, albeit the one most people are familiar with. Another popular standard is called Atom. You will notice that most websites created by software programs (especially blog software like WordPress, Typepad, etc.) offer both Atom and RSS feeds. Most (all) feed readers can accept either and the average consumer of feeds couldn’t care less about this technical detail.