I will review at an elementary level the ideas involved in decoherence. This will be done with liberal use of examples, taken from quantum optics, solid-state physics (superconductors, spin systems, nanoelectronics), and quantum computation. I will explain the 2 basic kinds of theoretical model (using spin and oscillator baths) that are used to study decoherence in all these systems, emphasizing how these can be related to experiment, and also the open questions in the field. Since this is supposed to be a fairly general talk I will also discuss how questions about decoherence are related to larger issues in the foundations of quantum mechanics and non-equilibrium physics; and briefly describe some of the speculations surrounding the role of decoherence in biology and cosmology.