What is a globular cluster? An observational perspective
  Globular clusters are large and dense agglomerate of stars. At variance with
smaller clusters of stars, they exhibit signs of some chemical evolution. At
least for this reason, they are intermediate between open clusters and massive
objects such as nuclear clusters or compact galaxies. While some facts are well
established, the increasing amount of observational data are revealing a
complexity that has so far defied the attempts to interpret the whole data set
in a simple scenario. We review this topic focusing on the main observational
features of clusters in the Milky Way and its satellites. We find that most of
the observational facts related to the chemical evolution in globular clusters
are described as being primarily a function of the initial mass of the
clusters, tuned by further dependence on the metallicity - that mainly affects
specific aspects of the nucleosynthesis processes involved - and on the
environment, that likely determines the possibility of indepenedent chemical
evolution of the fragments or satellites, where the clusters form. We review
the impact of multiple populations on different regions of the colour-magnitude
diagram and underline the constraints related to the observed abundances of
lithium, to the cluster dynamics, and to the frequency of binaries in stars of
different chemical composition. We then re-consider the issues related to the
mass budget and the relation between globular cluster and field stars. Any
successful model of globular cluster formation should explain these facts.
