  It is becoming increasingly difficult for geometers and even physicists to
avoid papers containing phrases like `triangulated category', not to mention
derived functors. I will give some motivation for such things from algebraic
geometry, and show how the concepts are already familiar from topology. This
gives a natural and simple way to look at cohomology and other scary concepts
in homological algebra like Ext, Tor, hypercohomology and spectral sequences.
