Ruby is a programming language. You've probably never heard of it, but many San Franciscans spend their time installing rubygems and writing CRUD Apps all day long. Ruby and the Rails web framework were created as an elaborate ploy to make writing business websites that talk to a database seem cool and interesting. Some notable San Francisco startups using ruby:
Ruby is the most popular language in San Francisco among github users. Compare that to Oakland, which is more of a javascript town.
Rails is omasake, which is hipster-speak for "paternalistic" but rubyists prefer "opinionated" as if that is any better which it isn't. An understanding of ruby will give you deep insight into the Macbook Air-toting tech hipster psyche: independent-minded iconoclasts who all dress, think, and program exactly the same way.
