What is difference between validator widget and Expression widget.

Have a look in the Help Centre Documentation - http://support.ptc.com/cs/help/thingworx_hc/thingworx_6.6_hc/

In the Section "Mashup Builder" there is a "Widgets" subsection where there are descriptions and instructions for all of the Out Of The Box Widgets.

The Validator widget works like the Expression widget but return only a value True or False.