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 \leq 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 _ { \odot } , 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 _ { \odot } 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 .