Pareto Principle: the Pareto Principle, also known as the '80:20 Rule' states that 80% of efforts that are not time managed or unfocused generates only 20% of the desired output. However, 80% of the desired output can be generated using only 20% of a well time managed effort.
Sources and additional info:
Links:
[1] http://pulse.veltsos.com/category/it/project-management
[2] http://pulse.veltsos.com/category/it/time-management
[3] http://linux.philosweb.com/drupal5/files/tm.jpg
[4] http://www.projectsmart.co.uk/six-time-management-tips-for-project-managers.html
[5] http://blogs.techrepublic.com.com/project-management/?p=364
[6] http://en.wikipedia.org/wiki/Pareto_principle