YAML™ is a readable text format for data structures. As you'll see below, YAML can handle many common data types and structures. And what YAML can't handle natively can be supported through flexible type families. For example, YAML for Ruby uses type families to support storage of regular expressions, ranges and object instances.
For Ruby developers, YAML is a natural fit for object serialization and general data storage. Really, it's quite fantastic. Spreads right on your Rubyware like butter on bread!