  The discovery of the first diffuse interstellar bands (DIBs) dates back to
the pioneering years of stellar spectroscopy. Today, we know about 300
absorption structures of this kind. There exists a great variety of the
profiles and intensities of DIBs, so they can not be readily described,
classified or characterized. To the present day no reliable identification of
the DIBs' carriers has been found. Many carriers of DIBs have been proposed
over the years. They ranged from dust grains to free molecules of different
kinds, and to more exotic specimens, like hydrogen negative ion. Unfortunately,
none of them is responsible for observed DIBs. Furthermore, it was shown that a
single carrier cannot be responsible for all known DIBs. It is hard to estimate
how many carriers can participate in producing these bands. The problem is
further complicated by the fact that to this day it is still impossible to find
any laboratory spectrum of any substance which would match the astrophysical
spectra. Here, a historical outline concerning DIBs is followed by a brief
description of their whole population. Then, a special attention is focused on
the procedures trying to extract spectroscopic families within the set of all
known DIBs.
