The SmartSites job is to produce HTML pages.
The page producer is the “SmartSites Template Engine” and is based upon Apache Velocity.
Smart Sites Template Engine is the Core of Smart Sites.
The core engine is base upon Apache Velocity implementation.
Smart Sites (and BEEing Platform) adds to Velocity Engine some custom features and behaviour.
However, most of template engine code is written in Velocity template language.
For detailed documentation about Velocity template language you can refer to Official Velocity Documentation.
If you prefer a PDF format of Velocity Documentation, you can download from here.