Elementary is mostly composed of widgets, but it also contains some infrastructure modules that facilitate the use of widgets. These are the infrastructure modules:

• App
• Caches
• Cursors
• Debug
• Elementary Config
• Elementary Engine
• Elementary Fonts
• Elementary Profile
• Fingers
• Focus
• General
• Gesture Layer
• Password show last
• elm-prefs-data
• Getting Started
• Elementary Scrolling
• Scrollhints
• Mirroring
• Widget Scaling
• Styles
• Theme
• Transit
• Widget Tree Navigation

The other two main groups in Elementary are:

• Widget list - These are the widgets you'll compose your UI out of.
• Containers List - These are the containers which hold the widgets.

Also see other generic EFL programming guides:

• Threading