Nonabelian D-branes, Open Strings, and Gauge Theory

There is a subtle difference between the open string dynamics determined by the original dual resonance models and that determined by D-brane constructions within critical closed string theory. For instance, in contrast to the former, the latter have massless scalars in addition to the massless gluon shared by both. We introduce and explain the concept of nonabelian D-branes which illuminates this distinction. We employ this concept to offer new possibilities for string duals for large N QCD (pure Yang-Mills gauge theory).