  We present the most extensive and complete study of the properties for the
largest sample (46 objects) of gamma-ray burst (GRB) host galaxies. The
redshift interval and the mean redshift of the sample are 0<z<6.3 and z=0.96
(look-back time: 7.2 Gyr), respectively; 89% of the hosts are at z <~ 1.6.
Optical-near-infrared (NIR) photometry and spectroscopy are used to derive
stellar masses, star formation rates (SFRs), dust extinctions and
metallicities. The average stellar mass is 10^9.3 M_sun, with a 1 sigma
dispersion of 0.8 dex. The average metallicity for a subsample of 17 hosts is
about 1/6 solar and the dust extinction in the visual band (for a subsample of
10 hosts) is A_V=0.5. We obtain new relations to derive SFR from [OII] or UV
fluxes, when Balmer emission lines are not available. SFRs, corrected for dust
extinction, aperture-slit loss and stellar Balmer absorption are in the range
0.01-36 M_sun yr^-1. The median SFR per unit stellar mass (specific SFR) is 0.8
Gyr^-1. Equivalently the inverse quantity, the median formation timescale is
1.3 Gyr. Most GRBs are associated with the death of young massive stars, more
common in star-forming galaxies. Therefore GRBs are an effective tool to detect
star-forming galaxies in the universe. Star-forming galaxies at z<1.6 are a
faint and low-mass population, hard to detect by conventional optical-NIR
surveys, unless a GRB event occurs. There is no compelling evidence that GRB
hosts are peculiar galaxies. More data on the subclass of short GRB are
necessary to establish the nature of their hosts.
