  Aging phenomena have been studied in very different materials like polymers,
supercooled liquids or disordered orientational crystals. We recall here the
main features of aging in spin glasses, and use this example of magnetic
systems as a guideline for the description of the others. A particular
attention is put on the sensitivity of aging to the cooling rate and to
temperature variations. This allows us to point out differences between
temperature specific processes, yielding ``rejuvenation and memory effects'' as
known in spin glasses, and domain growth processes, giving cumulative
contributions at different temperatures. The relevance of wall depinning
processes to rejuvenation and memory effects is discussed at the light of
recent results on disordered ferromagnets.
