In the hierachy of geometrical objects gerbes are next to line bundles. They are chacterized by an integral closed 3-form. This is in string theory represented by the so called H-field. Gerbe modules are generalizations of vector bundles and appear naturally in string theory as D-branes. Their stable equivalence classes can be used to define twisted K-theory. In the case of the Wess-Zumino-Witten model the (equivariant) twisted K-theory is closely related to the Verlinde algebra.