  In this review we show how K-theory classifies RR-charges in type II string
theory and how the inclusion of the B-field modifies the general structure
leading to the twisted K-groups. Our main purpose is to give an expository
account of the physical relevance of K-theory and, in order to make it, we
consider different points of view: processes of tachyon condensation,
cancellation of global anomalies and gauge fixings. As a field to test the
proposals of K-theory, we concentrate on the study of the D6-brane, now seen as
a non-abelian monopole.
