This is a collection of pedagogical lectures and research papers that were presented during a combined course/conference program held at the International Centre for Theoretical Physics in Trieste in the summer of 1991. The lectures begin from an elementary level and were intended to bring student participants to the point where they could appreciate the research conference that came at the end of program.

This is a collection of pedagogical lectures and research papers that were presented during a combined course/conference program held at the International Centre for Theoretical Physics in Trieste in the summer of 1991. The lectures begin from an elementary level and were intended to bring student participants to the point where they could appreciate the research conference that came at the end of program.