  Gamma-ray bursts are still not fully understood events. However, their
exploration could provide a useful tool for a better understanding of the early
Universe because they belong to the most distant and violent objects that
astronomers know. This thesis tries to bring more information about a so-called
group of intermediate-duration bursts claimed by different authors employing
different data samples. Firstly, duration and spectral hardness properties of
bursts from the Reuven Ramaty High-Energy Solar Spectroscopic Imager are
statistically analysed. The obtained results bring a suspicion that these
intermediate bursts gather into a separate group. Secondly, these bursts are
investigated in more detail with respect to their spectral lags, peak count
rates, redshifts, supernova observations, and so forth. Thirdly, long-duration
bursts with known redshifts and with derived pseudo-redshifts detected by The
Burst and Transient Source Experiment, Swift and Fermi bursts with known
redshifts, are used to study the cosmological effects on the observed flux and
fluence distributions.
