Systems thinking is a field of study that is closely related to the field of system dynamics. The two fields share many of the same concepts and tools, but systems thinking is less concerned with computer modeling and simulation of systmes than it is with focusing its lens on "softer," less quantitative problems.
Systems thinking has found a niche within organizational dynamics, futures studies, and other fields that can benefit from the tools of structural analysis but typically consider problem domains that don't quantify well.
In 1990, Dr. Peter Senge, Director of the Organizational Learning Center at MIT, wrote The Fifth Discipline, which can arguably called the "bible" of systems thinking field.