I read them on YAML-wikipedia but not really understood the main difference between them. I saw there are someone using .yaml extension, however, Symfony2 use .yml extension.
YAML is a human-readable data serialization format that takes concepts from programming languages such as C, Perl, and Python, and ideas from XML and the data format of electronic mail.
YAML is a recursive acronym for “YAML Ain’t Markup Language”. Early in its development, YAML was said to mean “Yet Another Markup Language”,[3] but it was then reinterpreted (backronyming the original acronym) to distinguish its purpose as data-oriented, rather than document markup.
So, how exactly the different between .yaml and .yml? When should we prefer one over the other?