Observational properties of diffuse radio sources in Galaxy Clusters.
  Current knowledge and open questions
  Diffuse radio sources in galaxy clusters are unique signposts of cluster
assembly in the Universe. Our knowledge of their observational properties has
considerably improved over the past decade, and the long standing questions
concerning their origin and rarity are now starting to receive some answers. It
is nowadays fairly well established that massive cluster mergers are the key
ingredient to account for the origin of halos and relics. The increasing number
of halos and relics found now allow to derive significant statistical
correlations. It has been shown that galaxy clusters may be radio-loud or
radio--quiet with respect to the presence of radio halos. Morevoer, radio halos
and relics are present only in unrelaxed clusters, whereas no such sources have
ever been found in relaxed systems. It has also become clear that a large
distribution exists for the spectral index of the synchrotron spectrum of radio
halos, and ultra-steep halos have been found.
  Despite all this, a number of observational issues remain open. Well defined
radio spectra, from ~ hundred MHz to GHz frequencies and good spectral imaging
are available only for few halos and relics, and such information is crucial
for a detailed understanding of their origin. It is presently unclear if there
are basic differences between clusters with ultra steep spectrum radio halos
and those hosting "classical" ones, and it is unknown if less energetic mergers
result in an observational signature in the radio band. With the forthcoming
advent of LOFAR and of the SKA Pathfinders, we will soon be able to observe
galaxy clusters with a major improvement in the radio sensitivity and frequency
coverage, and we expect to be able to soon address some of these pressing
questions.
