YML is a text file in data serialization language, YAML (Yet Another Markup Language), that is often used to create configuration files. It can work in combination with any programming language including C/C++, Ruby, Python, Java, PHP, C#, and others. Being independent of any specific programming language, it is open, interoperable, and human-readable. YML files can be opened and edited with common text editors such as Microsoft Notepad, Notepad++, Github Atom, Apple TextEdit, and Visual Studio IDE.
YML File Format - More Information
YML files store application settings as configuration. Its basic structure is a map that can also be called a directory or hash. Entries in a YML file consist of a key and value pair. Mostly, the configuration .yml files are saved as configure.yml, though some programming languages such as Ruby on Rails saves database connectivity configuration in database.yml files. YML/YAML file format specifications explain the complete language structures and syntax related Information.