Is library and set of tools to dynamically creating(generating) GUI's in different environments. Programs knows no think about widgets, etc. It can be compared to XUL and XSLT with Ecma Script, but it's designed to native applications.
Xul Dev means for Xul development environment. The IDE is intended to design Xul interfaces and to make programs in any programming language. The IDE is written in Scriptol C++ and available in source form and as binary executable.