  Gamma-ray bursts have been proved to be detectable up to distances much
larger than any other astrophysical object, providing the most effective way,
complementary to ordinary surveys, to study the high redshift universe. To this
end, we present here the results of an observational campaign devoted to the
study of the high-z GRB 090205. We carried out optical/NIR spectroscopy and
imaging of GRB 090205 with the ESO-VLT starting from hours after the event up
to several days later to detect the host galaxy. We compared the results
obtained from our optical/NIR observations with the available Swift high-energy
data of this burst. Our observational campaign led to the detection of the
optical afterglow and host galaxy of GRB 090205 and to the first measure of its
redshift, z=4.65. Similar to other, recent high-z GRBs, GRB 090205 has a short
duration in the rest-frame with T_{90,rf}=1.6 s, which suggests the possibility
that it might belong to the short GRBs class. The X-ray afterglow of GRB 090205
shows a complex and interesting behaviour with a possible rebrightening at
500-1000s from the trigger time and late flaring activity. Photometric
observations of the GRB 090205 host galaxy argue in favor of a starburst galaxy
with a stellar population younger than ~ 150 Myr. Moreover, the metallicity of
Z > 0.27 Z_Sun derived from the GRB afterglow spectrum is among the highest
derived from GRB afterglow measurement at high-z, suggesting that the burst
occurred in a rather enriched envirorment. Finally, a detailed analysis of the
afterglow spectrum shows the existence of a line corresponding to Lyman-alpha
emission at the redshift of the burst. GRB 090205 is thus hosted in a typical
Lyman-alpha emitter (LAE) at z=4.65. This makes the GRB 090205 host the
farthest GRB host galaxy, spectroscopically confirmed, detected to date.
