YAML is a human-readable data-serialization language. It is commonly used for configuration files and in applications where data is being stored or transmitted. YAML targets many of the same communications applications as Extensible Markup Language but has a minimal syntax which intentionally differs from SGML. (Source: wikipedia)

.yml

topic:
  type:
    - type: type1
      name: name1
      fields:
        class: class1
        property: -C
    - type: type2
      name: nam1
      fields:
        class: class2
        property: -D

#This is a comment