From Forum Nokia Wiki

Hildon is a user interface library for the Maemo platform. It is based on GTK and offers the UI Toolkit widgets for Mobile Computers and Internet Tablets.

Hildon has a somewhat simpler toolkit than GTK, but still requires some GTK for initialization.

Documentation for Hildon 2.2 (Maemo 5)

• Hildon 2.2 UI Style Guide
• Hildon 2.2 Widget UI Specification
• Fremantle Master Layout Guide