It contains:

• Manifest
  • Readme.txt
  • manifest.xml (???)
• Front-end parts
  • Views
  • CSS
  • JS
• Backend
  • Controller (1 or more than 1)
  • Model (1 or more than 1)
• Libraries
  • Extesion Libraries (can be used cross extension)
  • Extension custom action library
• Configurations
  • Config files
  • SQL updates;
    • Install.sql
    • Remove.sql