  Within the wide class of disordered materials, spin glasses occupy a special
place because of their conceptually simple definition of randomly interacting
spins. Their modelling has triggered spectacular developments of
out-of-equilibrium statistical physics, as well analytically as numerically,
opening the way to a new vision of glasses in general. "Real" spin glasses are
disordered magnetic materials which can be very diverse from the chemist's
point of view, but all share a number of common properties, laying down the
definition of generic spin glass behaviour. This paper aims at giving to
non-specialist readers an idea of what spin glasses are from an
experimentalist's point of view, describing as simply as possible their main
features as they can be observed in the laboratory, referring to numerous
detailed publications for more substantial discussions and for all theoretical
developments, which are hardly sketched here. We strived to provide the readers
who are interested in other glassy materials with some clues about the
potential of spin glasses for improving their understanding of disordered
matter. At least, arousing their curiosity for this fascinating subject will be
considered a success.
